AttentionSetIT: fix errorprone warnings

Change-Id: I79e400563c92a2a4f69ff72fef307dc0b740ba09
diff --git a/javatests/com/google/gerrit/acceptance/rest/change/AttentionSetIT.java b/javatests/com/google/gerrit/acceptance/rest/change/AttentionSetIT.java
index 800ee42..03e4bfb 100644
--- a/javatests/com/google/gerrit/acceptance/rest/change/AttentionSetIT.java
+++ b/javatests/com/google/gerrit/acceptance/rest/change/AttentionSetIT.java
@@ -622,7 +622,7 @@
     assertThat(attentionSet).hasReasonThat().isEqualTo("reason");
 
     // No emails for adding to attention set were sent.
-    email.getMessages().isEmpty();
+    assertThat(email.getMessages()).isEmpty();
   }
 
   @Test
@@ -631,6 +631,7 @@
     // implictly adds the user to the attention set when adding as reviewer
     change(r).addReviewer(user.email());
     requestScopeOperations.setApiUser(user.id());
+    email.clear();
 
     ReviewInput reviewInput =
         ReviewInput.create().removeUserFromAttentionSet(user.email(), "reason");
@@ -643,7 +644,7 @@
     assertThat(attentionSet).hasReasonThat().isEqualTo("reason");
 
     // No emails for removing from attention set were sent.
-    email.getMessages().isEmpty();
+    assertThat(email.getMessages()).isEmpty();
   }
 
   @Test