fix #2968 issue with shallowFieldsStateRef remain empty with append action

[bluebill1049]

fixed csb: https://codesandbox.io/s/react-hook-form-usefieldarray-forked-z9mif?file=/src/src/useFieldArray.ts

• unit test

[github-actions]

Size Change: +152 B (0%)

Total Size: 94.2 kB

Filename	Size	Change
dist/index.cjs.development.js	15.1 kB	+18 B (0%)
dist/index.cjs.production.min.js	8.9 kB	+32 B (0%)
dist/index.esm.js	15.1 kB	+18 B (0%)
dist/index.ie11.development.js	19.2 kB	+25 B (0%)
dist/index.ie11.production.min.js	11.2 kB	+16 B (0%)
dist/index.umd.development.js	15.4 kB	+17 B (0%)
dist/index.umd.production.min.js	8.99 kB	+26 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
dist/index.ie11.js	143 B	0 B
dist/index.js	142 B	0 B

_{compressed-size-action}

[codesandbox-ci]

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Sandbox	Source
react-hook-form/react-hook-form: app	Configuration
React Hook Form - useFieldArray (forked)	PR