refresh meta info only computed $metaInfo exists

[heww]

see more ElemeFE/element#4207 (comment)

[codecov-io]

Codecov Report

Merging #97 into master will decrease coverage by 0.23%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #97      +/-   ##
==========================================
- Coverage   25.11%   24.88%   -0.24%     
==========================================
  Files          18       18              
  Lines         211      213       +2     
==========================================
  Hits           53       53              
- Misses        158      160       +2

Impacted Files	Coverage Δ
src/shared/plugin.js	5.26% <0%> (-0.62%)	⬇️

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