BF: handle uninstalled subdatasets more gracefully

[adswa]

Previously, an uninstalled subdataset would catch create-sibling-gitlab
off-guard, as if no path was given, its error handling would try to
iterate over a none-type object.
This change accounts for this possibility, and makes create-sibling-gitlab
handle this situation a bit more graceful and with better messaging.
Fixes #7429"

[codecov]

Codecov Report

Patch coverage: 80.00% and project coverage change: -0.24 ⚠️

Comparison is base (5bae877) 91.48% compared to head (7b076b3) 91.24%.

Additional details and impacted files

@@            Coverage Diff             @@
##            maint    #7430      +/-   ##
==========================================
- Coverage   91.48%   91.24%   -0.24%     
==========================================
  Files         325      325              
  Lines       43399    43408       +9     
  Branches     5818        0    -5818     
==========================================
- Hits        39703    39609      -94     
- Misses       3681     3799     +118     
+ Partials       15        0      -15     

Impacted Files	Coverage Δ
...ad/distributed/tests/test_create_sibling_gitlab.py	97.29% <66.66%> (-1.30%)	⬇️
datalad/distributed/create_sibling_gitlab.py	69.46% <100.00%> (+0.55%)	⬆️

... and 39 files with indirect coverage changes

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[adswa]

Note to self: add a test, rebase on maint

[yarikoptic]

• I think it could go against maint.
• Having a test would have been nice.
• Changelog should be adjusted to note that this is create-sibling-gitlab specific, not some general handling.

[codeclimate]

Code Climate has analyzed commit 8726175 and detected 1 issue on this pull request.

Here's the issue category breakdown:

Category	Count
Complexity	1

View more on Code Climate.

[yarikoptic]

Looks good. Let's proceed!

[yarikoptic-gitmate]

PR released in 0.19.3