BF: Don't claim content size is zero without status --annex availablity

[mih]

Before it would say:

% datalad status --annex basic
5961 annex'd files (0.0/10.1 MB present/total size)

Now it says:

% datalad status --annex basic
5961 annex'd files (10.1 MB recorded total size)

Full output with availability check remains unchanged:

% datalad status --annex availability
5961 annex'd files (89.7 KB/10.1 MB present/total size)

[codecov]

Codecov Report

Merging #3378 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3378      +/-   ##
==========================================
- Coverage   91.24%   91.22%   -0.03%     
==========================================
  Files         265      265              
  Lines       34452    34452              
==========================================
- Hits        31436    31429       -7     
- Misses       3016     3023       +7

Impacted Files	Coverage Δ
datalad/core/local/status.py	97.91% <ø> (ø)	⬆️
datalad/downloaders/http.py	83.73% <0%> (-2.78%)	⬇️

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[kyleam]

LGTM

(I touched up the commit message slightly before merging)