Make String::move of an invalidated String result in an invalidated String

[sandeepmistry]

Port of arduino/Arduino#5127 to SAMD core.

[ArduinoBot]

✅ Build completed.

⬇️ Build URL: http://downloads.arduino.cc/PR/samd/package_samd-b87_index.json

ℹ️ To test this build:

1. Open the Preferences of the Arduino IDE.
2. Add the Build URL above in the Additional Boards Manager URLs field, and click OK.
3. Open the Boards Manager (menu Tools->Board->Board Manager...)
4. Install Arduino SAMD core - Pull Request Make String::move of an invalidated String result in an invalidated String #153
5. Select one of the boards under SAMD Pull Request Make String::move of an invalidated String result in an invalidated String #153 in Tools->Board menu
6. Compile/Upload as usual