Update evalError test rule

Since #32, the behaviour of `to_ip()` changed and now it doesn't raise
an exception in those circumstances. Now we use `regex()` to throw an
exception instead.

src/test/java/org/graylog/plugins/pipelineprocessor/functions/FunctionsSnippetsTest.java

@@ -332,7 +332,7 @@ public void evalError() {
 
         assertThat(context).isNotNull();
         assertThat(context.hasEvaluationErrors()).isTrue();
-        assertThat(Iterables.getLast(context.evaluationErrors()).toString()).isEqualTo("In call to function 'to_ip' at 5:28 an exception was thrown: 'null' is not an IP string literal.");
+        assertThat(Iterables.getLast(context.evaluationErrors()).toString()).isEqualTo("In call to function 'regex' at 5:28 an exception was thrown: Argument 'value' cannot be 'null'");
     }
 
     @Test

src/test/resources/org/graylog/plugins/pipelineprocessor/functions/evalError.txt

@@ -2,5 +2,5 @@ rule "trigger null"
 when
   true
 then
-  set_field("this_is_null", to_ip($message.does_not_exist));
+  set_field("do_not_exist", regex(".*", to_string($message.do_not_exist)).matches);
 end