fix npe

Change-Id: I5d43f5683fa33d34a7192ad36c8efdf87d01f0c2

Author: igorbernstein2

google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/internal/compat/GaxBasicChannelProvider.java

@@ -40,7 +40,8 @@ public class GaxBasicChannelProvider implements ChannelProvider {
   @SuppressWarnings("rawtypes")
   public GaxBasicChannelProvider(
       InstantiatingGrpcChannelProvider inner, @Nullable Credentials credentials) {
-    this.channelConfigurator = inner.toBuilder().getChannelConfigurator();
+    this.channelConfigurator = Optional.ofNullable(inner.toBuilder().getChannelConfigurator())
+        .orElse(b -> b);
     this.inner = inner.toBuilder().setAttemptDirectPath(false).build();
     this.credentials = Optional.ofNullable(credentials).map(MoreCallCredentials::from).orElse(null);
   }