Panic when dereferencing non-object variables

[afflerbach]

Describe the bug

Panic when dereferencing non-object (string, array, possibly number as well) variables (as discussed in chat the other day):

ERRO[0032] not a string                                  build=dev handler=endpoint panic="
goroutine 53 [running]:
runtime/debug.Stack()
	/usr/lib64/go/1.17/src/runtime/debug/stack.go:24 +0x72
github.com/avenga/couper/handler.(*Endpoint).ServeHTTP.func1()
	handler/endpoint.go:85 +0x8e
panic({0x103cac0, 0x1317090})
	/usr/lib64/go/1.17/src/runtime/panic.go:1047 +0x266
github.com/zclconf/go-cty/cty.Value.AsString({{{0x13378d8, 0xc0001a645f}}, {0x1171720, 0xc0002836b0}})
	vendor/github.com/zclconf/go-cty/cty/value_ops.go:1256 +0x19e
github.com/zclconf/go-cty/cty.Value.AsValueMap({{{0x1337a28, 0xc00011e990}}, {0x102c9a0, 0xc00011e9a8}})
	vendor/github.com/zclconf/go-cty/cty/value_ops.go:1326 +0x13b
github.com/avenga/couper/eval.walk.func1({0xc0002b954b, 0x5})
	eval/value.go:267 +0x94
github.com/avenga/couper/eval.walk({{{0x1337a28, 0xc00011e990}}, {0x102c9a0, 0xc00011e9a8}}, {{{0x0, 0x0}}, {0x0, 0x0}}, {0xc00028d370, 0x1, ...})
	eval/value.go:274 +0x215
github.com/avenga/couper/eval.walk({{{0x13379b8, 0xc00021cb50}}, {0x107be00, 0xc0002bf530}}, {{{0x0, 0x0}}, {0x0, 0x0}}, {0xc00028d360, 0x2, ...})
	eval/value.go:278 +0x285
github.com/avenga/couper/eval.walk({{{0x13379b8, 0xc00021cbd0}}, {0x107be00, 0xc0002bf680}}, {{{0x0, 0x0}}, {0x0, 0x0}}, {0xc00028d350, 0x3, ...})
	eval/value.go:278 +0x285
github.com/avenga/couper/eval.traversalValue(0xc0002d6818, {0xc00028d340, 0x4, 0x4})
	eval/value.go:254 +0x3a5
github.com/avenga/couper/eval.newLiteralValueExpr(0xc00011eb58, {0x1336fe0, 0xc000114d80})
	eval/value.go:127 +0x73d
github.com/avenga/couper/eval.newLiteralValueExpr(0xc00011eb58, {0x1336f00, 0xc00059a500})
	eval/value.go:106 +0x1385
github.com/avenga/couper/eval.Value(0x108f920, {0x1336f00, 0xc00052c140})
	eval/value.go:54 +0xc5
github.com/avenga/couper/handler/producer.NewResponse(0xc000099000, {0x1336db0, 0xc0002364d0}, 0xc000596b00, 0xc8)
	handler/producer/response.go:58 +0x898
github.com/avenga/couper/handler.(*Endpoint).ServeHTTP(0xc0003a78c0, {0x1330c78, 0xc0005a9200}, 0xc000099000)
	handler/endpoint.go:148 +0x319e
github.com/avenga/couper/handler.(*AccessControl).ServeHTTP(0xc0003a7950, {0x1330c78, 0xc0005a9200}, 0xc000099000)
	handler/access_control.go:36 +0x69c
github.com/avenga/couper/server.(*HTTPServer).ServeHTTP(0xc0003d2780, {0x1330c78, 0xc0005a9200}, 0xc000099000)
	server/http.go:242 +0x12ff
github.com/avenga/couper/handler/middleware.(*TraceHandler).ServeHTTP(0xc00007e430, {0x1330c78, 0xc0005a9200}, 0xc000099000)
	handler/middleware/trace.go:48 +0xb55
github.com/avenga/couper/handler/middleware.(*UID).ServeHTTP(0xc000420090, {0x1330c78, 0xc0005a9200}, 0xc000099000)
	handler/middleware/uid.go:56 +0xa07
github.com/avenga/couper/logging.(*AccessLog).ServeHTTP(0xc0003f7350, {0x1330c78,0xc0005a9200}, 0xc000099000, {0x131f120, 0xc000420090})
	logging/access_log.go:42 +0x97
github.com/avenga/couper/server.New.func1({0x1330c78, 0xc0005a9200}, 0xc0002e7688)
	server/http.go:100 +0x6b
net/http.HandlerFunc.ServeHTTP(0xc0003ee3a0, {0x1330c78, 0xc0005a9200}, 0x18de540)
	/usr/lib64/go/1.17/src/net/http/server.go:2046 +0x4e
github.com/avenga/couper/handler/middleware.NewRecordHandler.func1.1({0x1331758, 0xc0005ae1c0}, 0xc000099000)
	handler/middleware/status.go:30 +0x65e
net/http.HandlerFunc.ServeHTTP(0xc0004200c0, {0x1331758, 0xc0005ae1c0}, 0x18de568)
	/usr/lib64/go/1.17/src/net/http/server.go:2046 +0x4e
github.com/avenga/couper/server.New.func2({0x1331758, 0xc0005ae1c0},0xc000098f00)
	server/http.go:104 +0x45f
net/http.HandlerFunc.ServeHTTP(0xc0003f7380, {0x1331758, 0xc0005ae1c0}, 0x1)
	/usr/lib64/go/1.17/src/net/http/server.go:2046 +0x4e
net/http.serverHandler.ServeHTTP({0xc0002beae0}, {0x1331758, 0xc0005ae1c0}, 0xc000098f00)
	/usr/lib64/go/1.17/src/net/http/server.go:2878 +0x89b
net/http.(*conn).serve(0xc00059a3c0, {0x1336bb8, 0xc0004202d0})
	/usr/lib64/go/1.17/src/net/http/server.go:1929 +0x12e5
created by net/http.(*Server).Serve
	/usr/lib64/go/1.17/src/net/http/server.go:3033 +0x848

To Reproduce
Steps to reproduce the behavior:

1. Which Couper version?
   master

2. Provide your configuration file *.hcl. Remove sensitive data.
   see Failing test (crashes) when dereferencing primitive or array-type val… #376

…
value1 = request.query.foo[0].ooops
value2 = request.query.foo.ooops
…

3. Provide a curl call for reproduction
   $ curl -i 'localhost:8080/bad/dereference/array?foo=bar'

4. See an error response or error log.
   HTTP/1.1 500 Internal Server Error, see trace above or
   https://github.com/avenga/couper/pull/376/checks#step:7:1937

Expected behavior
No panic, succeeding test case in #376
$ go test -v -race ./server/http_integration_test.go -run TestEndpoint_ResponseNilEvaluation

Screenshots
✘

Additional context
#376