Treat empty string DartPackage as no DartPackage specified

mojo/public/tools/bindings/generators/mojom_dart_generator.py

Index: mojo/public/tools/bindings/generators/mojom_dart_generator.py
diff --git a/mojo/public/tools/bindings/generators/mojom_dart_generator.py b/mojo/public/tools/bindings/generators/mojom_dart_generator.py
index 59396d5018c84c4e44ed0df46063d689cde3b622..32f223d4c2a4d17b0040f31aea69c12fd00ff91f 100644
--- a/mojo/public/tools/bindings/generators/mojom_dart_generator.py
+++ b/mojo/public/tools/bindings/generators/mojom_dart_generator.py
@@ -445,7 +445,10 @@ def ParseStringAttribute(attribute):
 
 def GetPackage(module):
   if module.attributes and 'DartPackage' in module.attributes:
-    return ParseStringAttribute(module.attributes['DartPackage'])
+    package = ParseStringAttribute(module.attributes['DartPackage'])
+    package = package.strip()
+    if package != '':
+      return package
   # Default package.
   return 'mojom'