🔧 Fix typing of need docname/lineno

[chrisjsewell]

Account for these being None for external needs, that have no source mapping within the project

Practically, if is_external is False then docname/lineno arestr, but if is_external is True then docname/lineno are None.
This internal relationship is difficult to encode in the type system though.
So here we always account for docname is None, even if we have already checked for is_external is False

[codecov]

Codecov Report

Attention: Patch coverage is 78.04878% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 85.94%. Comparing base (3343ea0) to head (9c023b2).

Files	Patch %	Lines
sphinx_needs/layout.py	58.33%	5 Missing ⚠️
sphinx_needs/directives/needfilter.py	50.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1134      +/-   ##
==========================================
+ Coverage   85.92%   85.94%   +0.01%     
==========================================
  Files          56       56              
  Lines        6531     6539       +8     
==========================================
+ Hits         5612     5620       +8     
  Misses        919      919              

Flag	Coverage Δ
pytests	85.94% <78.04%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.