Updated `idwt2` function to handle None values as zero-arrays (at least one of four coefficient arrays must be not None).
Updated `idwt2` function to handle None values as zero-arrays (at least one of four coefficient arrays must be not None).

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extra : convert_revision : svn%3A993dc4b6-72fc-0310-9b3e-91fa30ebc9a8/pywt/trunk%40102

michelp/pywt,michelp/pywt,grlee77/pywt,kwohlfahrt/pywt,nigma/pywt,nigma/pywt,kwohlfahrt/pywt,aaren/pywt,grlee77/pywt,rgommers/pywt,ThomasA/pywt,michelp/pywt,rgommers/pywt,Dapid/pywt,ThomasA/pywt,PyWavelets/pywt,aaren/pywt,kwohlfahrt/pywt,ThomasA/pywt,eriol/pywt,aaren/pywt,eriol/pywt,Dapid/pywt,eriol/pywt,rgommers/pywt,PyWavelets/pywt,Dapid/pywt,nigma/pywt,rgommers/pywt