[mac] Unmap keys output default characters instead of null

**Describe the bug**
Keys which have not been mapped to any characters produce Latin characters. It may be a good idea to nullify them. This happens to some keys (highlighted in the screenshot below) on AltGr layer.


**To Reproduce**
Steps to reproduce the behavior:
1. Go to TextEdit or Google Docs
2. Choose the keyboard
3. Type AltGr+Y, U, I, O, P, H, J, K, L, N and M
4. See error

**Expected behavior**
Unmapped keys should not output any character as it may be the intention of the keyboard developer.

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**Keyman for macOS:**
- OS: Windows 10 Pro 1909
- Keyman version: 13.0.17
- Target Application: TextEdit / Google doc

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**Keyboard**
- Keyboard name: Khmerism SBBIC ([Link to the keyboard](https://drive.google.com/file/d/1d1CtkWnUblZ8-AjLcECFxq-3EO5a-dXJ/view?usp=sharing))
- Keyboard version: 1.1
- Language name: Khmer