BUG: Do not assert module swig attribute for init_docstring
BUG: Do not assert module swig attribute for init_docstring

Following up on
https://github.com/InsightSoftwareConsortium/ITK/commit/183cb1db67c5dbd30a25f279f913a4d2da5ea0b6
an `assert hasattr(this_module, "swig")` was added, but is true for most
casses, but this assertion causes issues when calling
`*init_docstring()` in the module. Replace the assert with a
conditation.

Also, simplify the namespace `swig` initialization further on.

Closes #2064

LucasGandel/ITK,vfonov/ITK,BRAINSia/ITK,BRAINSia/ITK,thewtex/ITK,thewtex/ITK,richardbeare/ITK,Kitware/ITK,richardbeare/ITK,richardbeare/ITK,vfonov/ITK,LucasGandel/ITK,thewtex/ITK,vfonov/ITK,vfonov/ITK,LucasGandel/ITK,InsightSoftwareConsortium/ITK,BRAINSia/ITK,InsightSoftwareConsortium/ITK,thewtex/ITK,vfonov/ITK,BRAINSia/ITK,thewtex/ITK,richardbeare/ITK,Kitware/ITK,hjmjohnson/ITK,LucasGandel/ITK,LucasGandel/ITK,richardbeare/ITK,LucasGandel/ITK,Kitware/ITK,BRAINSia/ITK,thewtex/ITK,InsightSoftwareConsortium/ITK,hjmjohnson/ITK,Kitware/ITK,richardbeare/ITK,hjmjohnson/ITK,InsightSoftwareConsortium/ITK,InsightSoftwareConsortium/ITK,vfonov/ITK,hjmjohnson/ITK,Kitware/ITK,hjmjohnson/ITK,LucasGandel/ITK,LucasGandel/ITK,BRAINSia/ITK,richardbeare/ITK,vfonov/ITK,BRAINSia/ITK,InsightSoftwareConsortium/ITK,vfonov/ITK,thewtex/ITK,hjmjohnson/ITK,vfonov/ITK,InsightSoftwareConsortium/ITK,Kitware/ITK,Kitware/ITK,hjmjohnson/ITK