vtgate panics if there are no vttablets and vtctlds

[sougou]

Repro:
Bring up etcd
Bring up vtgate
Run mysql client and issue a show databases

runtime error: invalid memory address or nil pointer dereference
/usr/local/go/src/runtime/panic.go:212 (0x44cbd9)
        panicmem: panic(memoryError)
/usr/local/go/src/runtime/signal_unix.go:687 (0x44ca28)
        sigpanic: panicmem()
/home/sougou/dev/src/vitess.io/vitess/go/vt/vtgate/vcursor_impl.go:388 (0xb954f8)
        io/vitess/go/vt/vtgate.parseDestinationTarget: if destKeyspace == "" && len(vschema.Keyspaces) == 1 {
/home/sougou/dev/src/vitess.io/vitess/go/vt/vtgate/vcursor_impl.go:135 (0xb934ae)
        io/vitess/go/vt/vtgate.newVCursorImpl: keyspace, tabletType, destination, err := parseDestinationTarget(safeSession.TargetString, vschema)
/home/sougou/dev/src/vitess.io/vitess/go/vt/vtgate/plan_execute.go:98 (0xb85fa9)
        io/vitess/go/vt/vtgate.(*planExecute).execute: vcursor, err := newVCursorImpl(ctx, safeSession, comments, e.e, logStats, e.e.vm, e.e.resolver.resolver)
/home/sougou/dev/src/vitess.io/vitess/go/vt/vtgate/fallback_executor.go:40 (0xb84a34)
        io/vitess/go/vt/vtgate.(*fallbackExecutor).execute: qr, err := f.exA.execute(ctx, safeSession, sql, bindVars, logStats)
/home/sougou/dev/src/vitess.io/vitess/go/vt/vtgate/executor.go:162 (0xb7462c)
        io/vitess/go/vt/vtgate.(*Executor).Execute: result, err = e.exec.execute(ctx, safeSession, sql, bindVars, logStats)
/home/sougou/dev/src/vitess.io/vitess/go/vt/vtgate/vtgate.go:246 (0xb9abf2)
        io/vitess/go/vt/vtgate.(*VTGate).Execute: qr, err = vtg.executor.Execute(ctx, "Execute", NewSafeSession(session), sql, bindVariables)
/home/sougou/dev/src/vitess.io/vitess/go/vt/vtgate/plugin_mysql_server.go:137 (0xb876b7)
        io/vitess/go/vt/vtgate.(*vtgateHandler).ConnectionClosed: _, _, _ = vh.vtg.Execute(ctx, session, "rollback", make(map[string]*querypb.BindVariable))
/usr/local/go/src/runtime/panic.go:973 (0x435d35)
        gopanic: reflectcall(nil, unsafe.Pointer(d.fn), deferArgs(d), uint32(d.siz), uint32(d.siz))
/usr/local/go/src/runtime/panic.go:212 (0x44cbd9)
        panicmem: panic(memoryError)
/usr/local/go/src/runtime/signal_unix.go:687 (0x44ca28)
        sigpanic: panicmem()
/home/sougou/dev/src/vitess.io/vitess/go/vt/vtgate/vcursor_impl.go:388 (0xb954f8)
        io/vitess/go/vt/vtgate.parseDestinationTarget: if destKeyspace == "" && len(vschema.Keyspaces) == 1 {
/home/sougou/dev/src/vitess.io/vitess/go/vt/vtgate/vcursor_impl.go:135 (0xb934ae)
        io/vitess/go/vt/vtgate.newVCursorImpl: keyspace, tabletType, destination, err := parseDestinationTarget(safeSession.TargetString, vschema)
/home/sougou/dev/src/vitess.io/vitess/go/vt/vtgate/plan_execute.go:98 (0xb85fa9)
        io/vitess/go/vt/vtgate.(*planExecute).execute: vcursor, err := newVCursorImpl(ctx, safeSession, comments, e.e, logStats, e.e.vm, e.e.resolver.resolver)
/home/sougou/dev/src/vitess.io/vitess/go/vt/vtgate/fallback_executor.go:40 (0xb84a34)
        io/vitess/go/vt/vtgate.(*fallbackExecutor).execute: qr, err := f.exA.execute(ctx, safeSession, sql, bindVars, logStats)
/home/sougou/dev/src/vitess.io/vitess/go/vt/vtgate/executor.go:162 (0xb7462c)
        io/vitess/go/vt/vtgate.(*Executor).Execute: result, err = e.exec.execute(ctx, safeSession, sql, bindVars, logStats)
/home/sougou/dev/src/vitess.io/vitess/go/vt/vtgate/vtgate.go:246 (0xb9abf2)
        io/vitess/go/vt/vtgate.(*VTGate).Execute: qr, err = vtg.executor.Execute(ctx, "Execute", NewSafeSession(session), sql, bindVariables)
/home/sougou/dev/src/vitess.io/vitess/go/vt/vtgate/plugin_mysql_server.go:215 (0xb881d4)
        io/vitess/go/vt/vtgate.(*vtgateHandler).ComQuery: session, result, err := vh.vtg.Execute(ctx, session, query, make(map[string]*querypb.BindVariable))
/home/sougou/dev/src/vitess.io/vitess/go/mysql/conn.go:1136 (0x8730d7)
        io/vitess/go/mysql.(*Conn).execQuery: err := handler.ComQuery(c, query, func(qr *sqltypes.Result) error {
/home/sougou/dev/src/vitess.io/vitess/go/mysql/conn.go:819 (0x893382)
        io/vitess/go/mysql.(*Conn).handleNextCommand.func1: if err := c.execQuery(sql, handler, more); err != nil {
/home/sougou/dev/src/vitess.io/vitess/go/mysql/conn.go:827 (0x872e1a)
        io/vitess/go/mysql.(*Conn).handleNextCommand: }()
/home/sougou/dev/src/vitess.io/vitess/go/mysql/server.go:464 (0x88e1d0)
        io/vitess/go/mysql.(*Listener).handle: err := c.handleNextCommand(l.handler)
/usr/local/go/src/runtime/asm_amd64.s:1373 (0x468a80)

[harshit-gangal]

The issue is non-reproducible on current master.