Do not fail dropping an uninitialized AnnexRepo

[mih]

Previously it would fail trying to determine whether the local
annex is known to remotes, but before init there is no local
annex.

With this change I am catching this condition and avoid the entire
investigation.

The potentially bigger issue (whether or not such a dataset should
carry an AnnexRepo instance, rather than GitRepo) is related
to #6103 and must be discussed separately.

Fixes #6180

[codecov]

Codecov Report

Merging #6183 (3c6da5b) into master (f240bdb) will decrease coverage by 15.16%.
The diff coverage is 55.69%.

❗ Current head 3c6da5b differs from pull request most recent head abb2457. Consider uploading reports for the commit abb2457 to get more accurate results

@@             Coverage Diff             @@
##           master    #6183       +/-   ##
===========================================
- Coverage   89.70%   74.54%   -15.17%     
===========================================
  Files         317      317               
  Lines       41635    41628        -7     
===========================================
- Hits        37350    31032     -6318     
- Misses       4285    10596     +6311     

Impacted Files	Coverage Δ
datalad/core/distributed/tests/test_push.py	0.00% <0.00%> (-98.94%)	⬇️
datalad/core/local/tests/test_diff.py	0.00% <0.00%> (-99.54%)	⬇️
datalad/core/local/tests/test_results.py	0.00% <0.00%> (-100.00%)	⬇️
datalad/core/local/tests/test_run.py	16.71% <ø> (-83.29%)	⬇️
datalad/core/local/tests/test_status.py	0.00% <0.00%> (-98.48%)	⬇️
...ad/distributed/tests/test_create_sibling_ghlike.py	69.69% <0.00%> (ø)
...talad/distributed/tests/test_create_sibling_ria.py	37.50% <0.00%> (-62.50%)	⬇️
datalad/distribution/create_sibling.py	64.73% <ø> (ø)
datalad/distribution/tests/test_get.py	100.00% <ø> (ø)
datalad/interface/common_opts.py	100.00% <ø> (ø)
... and 168 more

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