Fix Function.get_window() crash when there is no OVER clause

[uttam12331]

Summary

Function.get_window() raises AttributeError for any function without an OVER clause:

>>> import sqlparse
>>> sqlparse.parse('foo(x)')[0].tokens[0].get_window()
AttributeError: 'NoneType' object has no attribute 'tokens'

token_next_by() returns an (idx, token) tuple — (None, None) when nothing matches. A tuple is always truthy, so the existing if not over_clause: guard is dead code and never returns None; execution falls through to over_clause[1].tokens[-1], and over_clause[1] is None.

Fix

Guard on the token instead of the tuple, matching the sibling get_parameters() in the same class (which uses token_next_by(i=Parenthesis)[1]):

over_clause = self.token_next_by(i=Over)
if over_clause[1] is None:
    return None
return over_clause[1].tokens[-1]

The OVER-present paths are unchanged (window token still returned); only the no-OVER case now correctly returns None.

Tests

Added a negative-path assertion to test_grouping_function (foo(5) → get_window() is None). Full suite passes locally (487 passed).