discard all empty segments set for merge

[PSeitz]

Return error in start merge and IndexMerger constructor, when all segments to merge are empty

fixes #1189

[codecov-commenter]

Codecov Report

Merging #1427 (9c2ef81) into main (ea72cf3) will increase coverage by 0.00%. The diff coverage is 90.67%.

@@           Coverage Diff           @@
##             main    #1427   +/-   ##
=======================================
  Coverage   93.97%   93.97%           
=======================================
  Files         242      242           
  Lines       44661    44749   +88     
=======================================
+ Hits        41968    42054   +86     
- Misses       2693     2695    +2     

Impacted Files	Coverage Δ
src/indexer/merger.rs	97.58% <ø> (ø)
src/indexer/segment_updater.rs	95.31% <75.55%> (-0.43%)	:arrow_down:
src/indexer/index_writer.rs	96.99% <100.00%> (+0.17%)	:arrow_up:
src/indexer/segment_manager.rs	85.43% <100.00%> (+2.88%)	:arrow_up:
src/indexer/segment_writer.rs	95.86% <0.00%> (-0.54%)	:arrow_down:
src/fastfield/multivalued/mod.rs	95.65% <0.00%> (+1.44%)	:arrow_up:

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