RFR: 8284281: [Accessibility] [Win] [Narrator] Exceptions with TextArea & TextField when deleted last char

[Ambarish Rapte]

Issue:
When Narrator is running, 
1. Deleting last character from `TextField` throws `IllegalArgumentException`, and 
2. Deleting last character from `TextArea` throws `NPE`.

Fix:
When character is deleted, we receive an offset larger by one than the current text length. This scenario needs to be handled correctly.
The change in `Text.java` fixes the NPE with TextArea, and,
The change in `WinTextRangeProvider.java` fixes the IllegalArgumentException with TextField.

To observe the issue.
1. Run any program with TextField and/or TextArea
3. Launch Windows Narrator
4. Delete the last character from TextField / TextArea
5. Observe the related Exception

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Commit messages:
 - TextField and TextArea last char del fix for a11y

Changes: https://git.openjdk.org/jfx/pull/884/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=884&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8284281
  Stats: 3 lines in 2 files changed: 1 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/884.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/884/head:pull/884

PR: https://git.openjdk.org/jfx/pull/884