Minor: Fix Iterable[SplitResultResidual] type errors (#15634)

* Fix Iterable[SplitResultResidual] type errors

* fixup! Fix Iterable[SplitResultResidual] type errors

Author: TheNeuralBit

sdks/python/apache_beam/runners/common.py

@@ -567,9 +567,10 @@ def invoke_process(self,
                      additional_args=None,
                      additional_kwargs=None
                     ):
-    # type: (...) -> None
+    # type: (...) -> Iterable[SplitResultResidual]
     self.output_processor.process_outputs(
         windowed_value, self.process_method(windowed_value.value))
+    return []
 
 
 class PerWindowInvoker(DoFnInvoker):

sdks/python/apache_beam/runners/worker/operations.py

@@ -710,13 +710,12 @@ def start(self):
   def process(self, o):
     # type: (WindowedValue) -> None
     with self.scoped_process_state:
-      delayed_application = self.dofn_runner.process(o)
-      if delayed_application:
+      delayed_applications = self.dofn_runner.process(o)
+      if delayed_applications:
         assert self.execution_context is not None
-        # TODO(BEAM-77746): there's disagreement between subclasses
-        #  of DoFnRunner over the return type annotations of process().
-        self.execution_context.delayed_applications.append(
-            (self, delayed_application))  # type: ignore[arg-type]
+        for delayed_application in delayed_applications:
+          self.execution_context.delayed_applications.append(
+              (self, delayed_application))
 
   def finalize_bundle(self):
     # type: () -> None