add allow-empty-input in linter

[diegovar]

add allow-empty-input in linter so that changes in storybook js don't fail in pre-commit

Prevents this:

husky > pre-commit (node v12.16.2)
✔ Preparing...
⚠ Running tasks...
  ✔ Running tasks for *.js
  ❯ Running tasks for *.(js|css)
    ✖ stylelint --fix [FAILED]
  ↓ No staged files match *.md [SKIPPED]
  ↓ No staged files match *.php [SKIPPED]
  ↓ No staged files match *.(xml|xml.dist) [SKIPPED]
  ↓ No staged files match *.yml [SKIPPED]
↓ Skipped because of errors from tasks. [SKIPPED]
✔ Reverting to original state because of errors...
✔ Cleaning up... 

✖ stylelint --fix:
Error: No files matching the pattern ".storybook/stories/playground/index.js" were found.
    at /Users/diegovar/Development/projects/web-stories-wp/node_modules/stylelint/lib/standalone.js:211:12
    at processTicksAndRejections (internal/process/task_queues.js:97:5)
husky > pre-commit hook failed (add --no-verify to bypass)

[codecov]

Codecov Report

Merging #1850 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1850   +/-   ##
=========================================
  Coverage     63.86%   63.86%           
  Complexity      235      235           
=========================================
  Files           588      588           
  Lines          9614     9614           
=========================================
  Hits           6140     6140           
  Misses         3474     3474           

[github-actions]

Size Change: 0 B

Total Size: 872 kB

ℹ️ View Unchanged

Filename	Size	Change
assets/css/edit-story.css	3.04 kB	0 B
assets/css/stories-dashboard.css	3.04 kB	0 B
assets/js/edit-story.js	406 kB	0 B
assets/js/stories-dashboard.js	460 kB	0 B

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