check for undefined label in _animateAndLockLabel()

[gentunian]

this._label should be checked for undefined in form-field.ts _animateAndLockLabel()

Reproduction

Go to: https://components-issue-gyskyq.stackblitz.io/

Steps to reproduce:

1. Toggle the slider
2. Check for console error: Error: Cannot read property 'nativeElement' of undefined

Expected Behavior

No errors in console.

Actual Behavior

Errors are throw in console: Error: Cannot read property 'nativeElement' of undefined

Environment

stackblitz example attached.

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