Fix a bug in _get_function_body_without_inners for module sdks.python.transforms.core

[DKER2]

Fixes #28061
Some functions are defined in a class like this:

class A:
     @Tagging
     def process():

The current function does not remove @tagging because the line begins with a space, rather than directly with the @ symbol. This will lead to miss-warning of class apache_beam.dataframe.convert.DataFrameToRowsFn

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[tvalentyn]

@DKER2 - which issues is this fixing? I think the description mentions an unrelated issue.

[tvalentyn]

ah, nvm, I understand it now.

[tvalentyn]

Thank you, @DKER2 !