Update PipelineTest.java

Author: milaGGL

firebase-firestore/src/androidTest/java/com/google/firebase/firestore/PipelineTest.java

@@ -3157,6 +3157,8 @@ public void testIfNull() {
 
   @Test
   public void testCoalesce() {
+    assumeFalse("Coalesce is not supported against the emulator.", isRunningAgainstEmulator());
+
     Task<Pipeline.Snapshot> execute =
         firestore
             .pipeline()