Fix error when no parenthesis in export statement

The previous, verbose code actually crashed when for example someone forgets to put parenthesis:

#pythran export test
def test():
    ...

In that case the p token passed to p_error is None, and so it crashes when trying to access p.value

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#	modified:   pythran/spec.py
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#	doc/PYTHONIC
#	pythran/tests/rosetta/prime_decomposition.cpp
#	pythran/tests/rosetta/test.cpp
#	pythran/tests/rosetta/test.py