Repository name test fails if url contains a trailing slash

[LennyN95]

The folowing test line fails if the url ends with a trailing '/'.

https://github.com/Slicer/ExtensionsIndex/blob/466d0f864072966f730d5f2d9c357a16c213c284/scripts/check_description_files.py#LL84C5-L84C95

I'd suggest we either remove trailing slashes or, if trailing slashes are discouraged, we could implement a check and fail with a meaningful message, because the current one suggests the repository name is empty which could be misleading.