fix(pregel): handle EmptyInputError for Command with stream_mode='values'

[nicolasmota]

I'm facing some errors related to undeclared resume_is_map variable.

  File "/app/venv/lib/python3.11/site-packages/langgraph/pregel/main.py", line 3179, in ainvoke
    async for chunk in self.astream(
  File "/app/venv/lib/python3.11/site-packages/langgraph/pregel/main.py", line 2914, in astream
    async with AsyncPregelLoop(
  File "/app/venv/lib/python3.11/site-packages/langgraph/pregel/_loop.py", line 1344, in __aenter__
    self.updated_channels = self._first(
  File "/app/venv/lib/python3.11/site-packages/langgraph/pregel/_loop.py", line 722, in _first
    if not writes and not resume_is_map:
UnboundLocalError: cannot access local variable 'resume_is_map' where it is not associated with a value

• This PR fixes this error declaring the default value for this Variable being possible to throw the error:EmptyInputError
• Added tests to ensure that invoking Command() with stream_mode='values' correctly raises EmptyInputError when input is empty, and that Command(resume=...) works as expected.
• Updated imports to include EmptyInputError in relevant test files.

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