Remove from empty indexes

[olenagerasimova]

Closes #415
Corrected RpmMetadata.Remove() to work with empty indexes properly.

[codecov]

Codecov Report

Merging #417 (ca3f06f) into master (533e9f4) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #417      +/-   ##
============================================
- Coverage     84.22%   84.20%   -0.03%     
  Complexity      305      305              
============================================
  Files            56       56              
  Lines          1864     1861       -3     
  Branches        103      103              
============================================
- Hits           1570     1567       -3     
  Misses          276      276              
  Partials         18       18              

Impacted Files	Coverage Δ
...main/java/com/artipie/rpm/meta/XmlPrimaryMaid.java	91.07% <100.00%> (-0.46%)	⬇️

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[g4s8]

@olenagerasimova thanks