style(meta): remove NothingMutation

[xxchan]

What's changed and what's your intention?

use None instead

Checklist

• [ ] I have written necessary docs and comments
• [ ] I have added necessary unit tests and integration tests
• All checks passed in ./risedev check (or alias, ./risedev c)

Refer to a related PR or issue link (optional)

[codecov]

Codecov Report

Merging #3256 (1ebdd57) into main (4f0ef58) will increase coverage by 0.00%.
The diff coverage is 21.00%.

@@           Coverage Diff           @@
##             main    #3256   +/-   ##
=======================================
  Coverage   73.22%   73.22%           
=======================================
  Files         747      747           
  Lines      101892   101892           
=======================================
+ Hits        74611    74614    +3     
+ Misses      27281    27278    -3     

Flag	Coverage Δ
rust	73.22% <21.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/meta/src/stream/source_manager.rs	26.23% <0.00%> (ø)
src/stream/src/executor/mod.rs	49.65% <17.85%> (+1.02%)	⬆️
src/meta/src/barrier/command.rs	75.59% <100.00%> (ø)
src/meta/src/barrier/mod.rs	70.23% <100.00%> (ø)

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[yezizp2012]

LGTM