feat: add error check for null response from API

gax/src/fallbackRest.ts

@@ -85,14 +85,19 @@ export function decodeResponse(
 ): {} {
   // eslint-disable-next-line n/no-unsupported-features/node-builtins
   const decodedString = new TextDecoder().decode(response);
+  if (!decodedString) {
+    throw new Error(`Received null response from RPC ${rpc.name}`);
+  }
   const json = JSON.parse(decodedString);
   if (!ok) {
     const error = GoogleError.parseHttpError(json);
     throw error;
   }
   const message = serializer.fromProto3JSON(rpc.resolvedResponseType!, json);
   if (!message) {
-    throw new Error(`Received null response from RPC ${rpc.name}`);
+    throw new Error(
+      `Received null or malformed response from JSON serializer from RPC ${rpc.name}`
+    );
   }
   return rpc.resolvedResponseType!.toObject(message, defaultToObjectOptions);
 }

gax/test/unit/grpc-fallback.ts

@@ -356,6 +356,27 @@ describe('grpc-fallback', () => {
       });
     });
   });
+  it('should handle a null response from the API ', done => {
+    const requestObject = {content: 'test-content'};
+    const expectedMessage = 'Received null response from RPC Echo';
+
+    //@ts-ignore
+    sinon.stub(nodeFetch, 'Promise').returns(
+      Promise.resolve({
+        ok: false,
+        arrayBuffer: () => {
+          return Promise.resolve(Buffer.from(''));
+        },
+      })
+    );
+    gaxGrpc.createStub(echoService, stubOptions).then(echoStub => {
+      echoStub.echo(requestObject, {}, {}, (err?: Error) => {
+        assert(err instanceof Error);
+        assert.strictEqual(err.message, expectedMessage);
+        done();
+      });
+    });
+  });
 
   it('should handle a fetch error', done => {
     const requestObject = {content: 'test-content'};