append_doc method breaks with interpreter optimization on

[sevdog]

The CPython interpreter has an optimization flag -OO which discards assertions and docstrings.

pyexcel/pyexcel/_compact.py

Lines 68 to 73 in ee0722a

	def append_doc(value):
	def _doc(func):
	func.__doc__ = dedent(func.__doc__) + "\n" + value
	return func
	return _doc

The current implementation of append_doc method causes a TypeError if this optimization level is set, since the __doc__ attribute is None (cause it was not kept in the compiled module):

   File "/opt/venv/lib/python3.9/site-packages/pyexcel/_compact.py", line 90, in _doc
     func.__doc__ = dedent(func.__doc__) + "\n" + value         
   File "/usr/local/lib/python3.9/textwrap.py", line 430, in dedent                                                                                                
     text = _whitespace_only_re.sub('', text)         
 TypeError: expected string or bytes-like object

A solution could be this method to do nothing if func.__doc__ is None.