Redis.get_all_databases添加边界测试方法

[feiazifeiazi]

1. get_all_databases方法异常逻辑的地方 添加注释。
2. 添加单元测试方法-测试边界返回值：test_get_all_databases_with_empty_return_value
3. 添加单元测试方法-测试边界返回值：test_get_all_databases_with_less_than_15_dbs

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.89%. Comparing base (370d8c0) to head (6ab4e7a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2574      +/-   ##
==========================================
+ Coverage   76.86%   76.89%   +0.03%     
==========================================
  Files         117      117              
  Lines       16084    16106      +22     
==========================================
+ Hits        12363    12385      +22     
  Misses       3721     3721              

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