Check for None in __doc__ before changing it.

[pbrod]

Python code byte-compiled with -OO has doc-strings stripped out.

This creates problems when compiling different packages which changes the doc-strings by doing something like this:
doc += "additional text"
(when the docstring is 'None', this will fail).

mgcv_cubic_splines.py does this and thus creates problems when trying to bytecompile it with the -OO option using cx_freeze/py2exe.

In order to fix this, I would propose to change all the lines which say:
doc += CubicRegressionSpline.common_doc
with
doc =doc +CubicRegressionSpline.common_doc if doc else ''

See also:
http://bugs.python.org/issue23189#
http://stackoverflow.com/questions/22299532/unsupported-operand-types-for-nonetype-and-str-winappdbg-error-after-c