provider/rabbitmq: Allow users without tags

[jtopjian]

This commit makes the tags attribute optional for users. It also
handles cases when a user defines a tag as an empty string ("").

Fixes #12296

[stack72]

LGTM :)

% make testacc TEST=./builtin/providers/rabbitmq                                            130 ↵ ✹ ✭
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/04/20 20:09:21 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/rabbitmq -v  -timeout 120m
=== RUN   TestAccBinding_importBasic
--- PASS: TestAccBinding_importBasic (0.41s)
=== RUN   TestAccExchange_importBasic
--- PASS: TestAccExchange_importBasic (0.23s)
=== RUN   TestAccPermissions_importBasic
--- PASS: TestAccPermissions_importBasic (0.21s)
=== RUN   TestAccPolicy_importBasic
--- PASS: TestAccPolicy_importBasic (0.24s)
=== RUN   TestAccQueue_importBasic
--- PASS: TestAccQueue_importBasic (0.24s)
=== RUN   TestAccUser_importBasic
--- PASS: TestAccUser_importBasic (0.07s)
=== RUN   TestAccVhost_importBasic
--- PASS: TestAccVhost_importBasic (0.16s)
=== RUN   TestProvider
--- PASS: TestProvider (0.00s)
=== RUN   TestProvider_impl
--- PASS: TestProvider_impl (0.00s)
=== RUN   TestAccBinding
--- PASS: TestAccBinding (0.27s)
=== RUN   TestAccExchange
--- PASS: TestAccExchange (0.21s)
=== RUN   TestAccPermissions
--- PASS: TestAccPermissions (0.21s)
=== RUN   TestAccPolicy
--- PASS: TestAccPolicy (0.22s)
=== RUN   TestAccQueue
--- PASS: TestAccQueue (0.25s)
=== RUN   TestAccUser_basic
--- PASS: TestAccUser_basic (0.11s)
=== RUN   TestAccUser_emptyTag
--- PASS: TestAccUser_emptyTag (0.17s)
=== RUN   TestAccUser_noTags
--- PASS: TestAccUser_noTags (0.11s)
=== RUN   TestAccVhost
--- PASS: TestAccVhost (0.15s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/rabbitmq	3.255s

[ghost]

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