[TextFields] Return the empty string instead of nil in MDCTextField's accessibilityValue method to avoid VoiceOver defaulting to [super accessibilityValue].
[TextFields] Return the empty string instead of nil in MDCTextField's accessibilityValue method to avoid VoiceOver defaulting to [super accessibilityValue].

PiperOrigin-RevId: 316103878

material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios