fix LogBatch::is_empty()

[tabokie]

Adding an empty list of entries to log batch should not change the output of is_empty().

Signed-off-by: tabokie xy.tao@outlook.com

[codecov]

Codecov Report

Merging #212 (2ee09c3) into master (2fe0c36) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #212      +/-   ##
==========================================
- Coverage   96.98%   96.96%   -0.02%     
==========================================
  Files          28       28              
  Lines        7687     7704      +17     
==========================================
+ Hits         7455     7470      +15     
- Misses        232      234       +2     

Impacted Files	Coverage Δ
src/log_batch.rs	97.34% <100.00%> (+0.05%)	⬆️
src/consistency.rs	91.11% <0.00%> (-2.23%)	⬇️
src/codec.rs	98.34% <0.00%> (-0.34%)	⬇️

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