fix: fix httpjson executor (#1448)

Author: mutianf

gax-httpjson/src/main/java/com/google/api/gax/httpjson/ManagedHttpJsonChannel.java

@@ -153,7 +153,7 @@ public static class Builder {
     private Builder() {}
 
     public Builder setExecutor(Executor executor) {
-      this.executor = Preconditions.checkNotNull(executor);
+      this.executor = executor == null ? DEFAULT_EXECUTOR : executor;
       return this;
     }
 

gax-httpjson/src/test/java/com/google/api/gax/httpjson/InstantiatingHttpJsonChannelProviderTest.java

@@ -62,6 +62,13 @@ public void basicTest() throws IOException {
     provider = provider.withEndpoint(endpoint);
     assertThat(provider.needsEndpoint()).isFalse();
 
+    assertThat(provider.needsHeaders()).isTrue();
+    provider = provider.withHeaders(Collections.<String, String>emptyMap());
+    assertThat(provider.needsHeaders()).isFalse();
+
+    // Make sure getTransportChannel works without setting executor
+    assertThat(provider.getTransportChannel()).isInstanceOf(HttpJsonTransportChannel.class);
+
     assertThat(provider.needsExecutor()).isTrue();
     provider = provider.withExecutor((Executor) executor);
     assertThat(provider.needsExecutor()).isFalse();
@@ -71,10 +78,6 @@ public void basicTest() throws IOException {
     provider = provider.withExecutor(executor);
     assertThat(provider.needsExecutor()).isFalse();
 
-    assertThat(provider.needsHeaders()).isTrue();
-    provider = provider.withHeaders(Collections.<String, String>emptyMap());
-    assertThat(provider.needsHeaders()).isFalse();
-
     assertThat(provider.acceptsPoolSize()).isFalse();
     Exception thrownException = null;
     try {