RichText: fix paste handler crash when pasting files

[jsnajdr]

Fixes a crash in RichText paste handler when files are pasted.

Steps to reproduce:

1. Create a core/pullquote block and focus the value rich text field
2. Try to paste a file into the field. For example, "Copy" a file in Finder, and in the focused field, select "Paste" from the right-click context menu.

The Pullquote block cannot accept a file paste: the value field can only accept a text, and unlike Paragraph or Heading, it doesn't support splitting. The useWritingFlow/useClipboardHandler doesn't do the paste, and the event falls through to the RichText default handler, which crashes because there is no text (plainText/html) in the event.

This PR fixes the crash by adding a guard.

[github-actions]

Size Change: 0 B

Total Size: 7.76 MB

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[Mamaduka]

Good catch, @jsnajdr!

[github-actions]

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Co-authored-by: jsnajdr <jsnajdr@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>

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