fix: Only send mtlsEndpoint if it is non-null (#3767)

Temporary fix to only plumb the mtlsEndpoint to the gRPCChannelProviders
if this has been set with a non-null value. Validation will still go
through if it is set with bogus values.

Issue reported in googleapis/java-bigtable#2565

This seems to due to how Bigtable wrappers the generated stubs. BigTable
manually calls setEndpoint to for the StubSettings, but does not do the
equivalent for setMtlsEndpoint. The EndpointContext is only aware of the
TLS endpoint and passes a null value to the ChannelProviders.

Potential longer term fix: BigTable's wrappers would need to set the
mTLS Endpoint. This would be needed for BigTable to support S2A. This
does not impact any of BigTable's existing functionality as the endpoint
resolution for all non-S2A flows is done via `setEndpoint()`.

Author: lqiu96

gax-java/gax/src/main/java/com/google/api/gax/rpc/ClientContext.java

@@ -223,7 +223,7 @@ public static ClientContext create(StubSettings settings) throws IOException {
       transportChannelProvider = transportChannelProvider.withEndpoint(endpoint);
     }
     transportChannelProvider = transportChannelProvider.withUseS2A(endpointContext.useS2A());
-    if (transportChannelProvider.needsMtlsEndpoint()) {
+    if (transportChannelProvider.needsMtlsEndpoint() && endpointContext.mtlsEndpoint() != null) {
       transportChannelProvider =
           transportChannelProvider.withMtlsEndpoint(endpointContext.mtlsEndpoint());
     }