fix: make meta not optional

[unional]

While technically meta can be undefined when not used (to save space).
Making it optional means every time you use it you need to check if it is undefined.

And since you use it, you assume that your specific action will have meta defined.

The same argument as redux-utilities/flux-standard-action#53

[coveralls]

Pull Request Test Coverage Report for Build 30

• 0 of 0 (NaN%) changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 29:	0.0%
Covered Lines:	13
Relevant Lines:	13

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💛 - Coveralls

[codecov]

Codecov Report

Merging #10 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #10   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           3      3           
  Lines          13     13           
  Branches        2      2           
=====================================
  Hits           13     13

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[unional]

🎉 This PR is included in version 1.5.1 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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