Fix crash in consul backend

[jbardin]

A TLS config was being assigned to a Transport in a nil http.Client. The
Transport is built in the consul config by default, but the http.Client
is not built until later in NewClient.

No new tests, because the existing tests were sufficient to catch this.
This was introduced in #15405, but there were no ACC tests run.

Fixes #15972

[jbardin]

=== RUN   TestBackend_impl
--- PASS: TestBackend_impl (0.00s)
=== RUN   TestBackend
--- PASS: TestBackend (4.17s)
	backend_test.go:55: TestBackend: testing state locking for *consul.Backend
=== RUN   TestBackend_lockDisabled
--- PASS: TestBackend_lockDisabled (3.16s)
	backend_test.go:78: TestBackend: testing state locking for *consul.Backend
	backend_test.go:78: TestBackend: *consul.Backend: empty string returned for lock, assuming disabled
=== RUN   TestBackend_gzip
--- PASS: TestBackend_gzip (2.55s)
=== RUN   TestRemoteClient_impl
--- PASS: TestRemoteClient_impl (0.00s)
=== RUN   TestRemoteClient
--- PASS: TestRemoteClient (2.48s)
=== RUN   TestRemoteClient_gzipUpgrade
--- PASS: TestRemoteClient_gzipUpgrade (2.15s)
=== RUN   TestConsul_stateLock
--- PASS: TestConsul_stateLock (3.74s)
=== RUN   TestConsul_destroyLock
--- PASS: TestConsul_destroyLock (1.88s)
=== RUN   TestConsul_lostLock
--- PASS: TestConsul_lostLock (2.42s)
PASS

[ghost]

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