Fix gwt:generate-module for relocated artifacts

Relocated artifacts don't have an associated file, despite
having a packaging=jar (so being considered "added to the
classpath" by their artifact handler).
This shouldn't happen much, because we expect people to
update their direct dependencies to their new location
(and gwt:generate-module only looks at direct dependencies),
but still, that's bad developer experience.

Fixes #150

Author: tbroyer

src/main/java/net/ltgt/gwt/maven/GenerateModuleMojo.java

@@ -248,6 +248,10 @@ private boolean generateInheritsFromDependencies(XMLWriter xmlWriter) throws IOE
         if (!artifact.getArtifactHandler().isAddedToClasspath()) {
           continue;
         }
+        if (artifact.getFile() == null) {
+          // This can happen for relocated artifacts
+          continue;
+        }
         realm.addURL(artifact.getFile().toURI().toURL());
       }
     } catch (DuplicateRealmException | MalformedURLException e) {