Add tests for babel-generator

[1egoman]

Q	A
Patch: Bug Fix?	no
Major: Breaking Change?	no
Minor: New Feature?	no
Deprecations?	no
Spec Compliancy?	yes
Tests Added/Pass?	yes
Fixed Tickets	#5326
License	MIT
Doc PR	no
Dependency Changes	no

Added two tests for src/generators/flow.js that check if a value is of
type empty or mixed. The goal was to cover https://codecov.io/gh/babel/babel/src/master/packages/babel-generator/src/generators/flow.js#L172 and https://codecov.io/gh/babel/babel/src/master/packages/babel-generator/src/generators/flow.js#L176, and after running ./scripts/test-cov.sh locally I confirm those lines are now covered.

[mention-bot]

@1egoman, thanks for your PR! By analyzing the history of the files in this pull request, we identified @ChauTNguyen, @bhosmer and @sugargreenbean to be potential reviewers.

[codecov]

Codecov Report

Merging #5845 into 7.0 will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              7.0    #5845   +/-   ##
=======================================
  Coverage   85.21%   85.21%           
=======================================
  Files         284      284           
  Lines        9958     9958           
  Branches     2780     2780           
=======================================
  Hits         8486     8486           
  Misses        971      971           
  Partials      501      501

Impacted Files	Coverage Δ
packages/babel-helper-call-delegate/src/index.js	64% <0%> (-4%)	⬇️
packages/babel-traverse/src/path/context.js	85.34% <0%> (-1.73%)	⬇️
packages/babel-traverse/src/path/modification.js	72.11% <0%> (-0.97%)	⬇️
packages/babel-traverse/src/visitors.js	85.71% <0%> (-0.96%)	⬇️
...bel-plugin-transform-es2015-classes/src/vanilla.js	91.06% <0%> (+0.42%)	⬆️
packages/babel-generator/src/generators/flow.js	99.26% <0%> (+1.47%)	⬆️

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[1egoman]

Looking at the coverage report above, it looks like the lines I intended to cover are covered:

Did I go about covering those lines in the right way?

[hzoo]

Yep, good work!