[experiments] Fix some bugs for no rollout definition (#33626)

Some bugs crept in if there's no rollout defined for an experiment, fix
that up.

tools/codegen/core/experiments_compiler.py

@@ -266,7 +266,7 @@ class ExperimentDefinition(object):
         return self._description
 
     def default(self, platform):
-        return self._default[platform]
+        return self._default.get(platform, False)
 
     @property
     def test_tags(self):
@@ -277,7 +277,7 @@ class ExperimentDefinition(object):
         return self._allow_in_fuzzing_config
 
     def additional_constraints(self, platform):
-        return self._additional_constraints[platform]
+        return self._additional_constraints.get(platform, {})
 
 
 class ExperimentsCompiler(object):