python-xmlsec

implement repr methods for __Transform and __KeyData classes, fix segfault for missing href attribute

Some constants are missing `href`, see e.g. [xmlSecTransformRemoveXmlTagsC14NKlass](https://github.com/lsh123/xmlsec/blob/cd6fe1c19bd40e0e3bd63af61c34d595371e2bc9/src/c14n.c#L697).
Access to `href` thus ends in a segfault in Python:

```
import xmlsec

print(xmlsec.constants.TransformRemoveXmlTagsC14N.href)
```

This PR fixes that by returning `None` to avoid a null pointer.

Signed-off-by: oleg.hoefling <oleg.hoefling@gmail.com>