在where中使用 is null 替换 = null

[zihuyishi]

mysql 中，where 语句后的 field = null 无效，应该使用 field is null

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@6f362c1). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master      #53   +/-   ##
=========================================
  Coverage          ?   86.11%           
=========================================
  Files             ?        6           
  Lines             ?      324           
  Branches          ?       51           
=========================================
  Hits              ?      279           
  Misses            ?       45           
  Partials          ?        0

Impacted Files	Coverage Δ
lib/operator.js	78.01% <100%> (ø)

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[fengmk2]

merge into #60