Fix for normpath for DXPath

[anujkumar93]

@jtratner @pkaleta

This PR fixes the normpath bug in DXVirtualPath and DXCanonicalPath

Background

Stor had a bug for DXPaths, wherein paths like DXPath('dx://project:/').normpath() errored with TypeError: cannot concatenate 'str' and 'NoneType' objects. This is because for such paths, the resource turns out to be None, which is then attempted to add it to a string '/' in normpath.

Changes

• Fixed the normpath function in DXVirtualPath and DXCanonicalPath through self.resource or ''
• Added tests for normpath

Tested via:

• Test suite was run successfully

[jtratner]

please add release note for this patch release