Detect compile_commands.json after containing folder deleted and recreated on Windows

[yiftahw]

closes #7030
On my setup (Windows 10 Home 22H2), when a folder is copied, it's containing files' stats.mtime remain the same (even though the containing folder's stats are new), which is the root cause of this issue.
For comparison on my WSL (Ubuntu 20.04 LTS), the new files' stats.mtime will be the time when the copied files were created.
The fix adds the check if the compileCommandsFile is undefined | null which means it was previously not "present".

note: a side effect of this fix is that on every initial load of a workspace, (if fs.stats didn't return an error) we fire the onCompileCommandsChanged event because CppProperties.compileCommandsFile is initially marked as undefined.
if we remove the undefined case from the condition check, the fix will only work if the compileCommandsFile was actually changed at least once (since the workspace was loaded) before the containing folder is deleted and re-created.

cases tested:

• containing folder deleted after workspace was loaded, and copied back on a Windows machine. (the issue, tested on Linux as well)
• containing folder doesn't exist and copied back after workspace was loaded on a Windows machine. (the issue, tested on Linux as well)
• compile_commands.json file deleted after workspace was loaded and re-created (with a new timestamp)
• compile_commands.json doesn't exist and generated after workspace is loaded.

[fearthecowboy]

Looks good to me