Fix NullPointerException at Java8StepDefinition.isDefinedAt

The api allows `StackTraceElement.getFileName()`[1] to return null. We
were not able to reproduce this problem reliably but it occurred in
production environments anyway.

Fixes #1217

References:
 1. docs.oracle.com/javase/7/docs/api/java/lang/StackTraceElement.html#getFileName()

Author: mpkorstanje

Diff for: java8/src/main/java/cucumber/runtime/java8/Java8StepDefinition.java

@@ -108,7 +108,7 @@ public void execute(final String language, final Object[] args) throws Throwable
 
     @Override
     public boolean isDefinedAt(StackTraceElement stackTraceElement) {
-        return location.getFileName().equals(stackTraceElement.getFileName());
+        return location.getFileName() != null && location.getFileName().equals(stackTraceElement.getFileName());
     }
 
     @Override