Fix unbinding keys in v0.10 (#4988)

## Summary of the Pull Request

We (the royal "we") broke key unbinding in #4746. We didn't run the local tests after this, which actually would have caught this. The comment even suggests what we should have done here. We need to make sure that when we bail, it's because there's a parsing function that returned nothing. `null`, `"unbound"`, etc actually don't even have a parsing function at all, so they should just keep on keepin' on.

## References

Source of this regression: #4746

## PR Checklist
* [x] Closes #3729
* [x] I work here
* [x] Tests added/passed
* [n/a] Requires documentation to be updated

## Detailed Description of the Pull Request / Additional comments

This is a great example of why your unittests should run in CI always

## Validation Steps Performed
* **ran the tests**
* tested the following unbindings:

```json
        { "command": null, "keys": [ "ctrl+shift+t" ] },
        { "command": "unbound", "keys": [ "ctrl+shift+t" ] },
        { "command": "null", "keys": [ "ctrl+shift+t" ] },

```
and they each individually worked.

src/cascadia/TerminalApp/AppKeyBindingsSerialization.cpp

@@ -516,7 +516,7 @@ std::vector<::TerminalApp::SettingsLoadWarnings> winrt::TerminalApp::implementat
                 warnings.insert(warnings.end(), parseWarnings.begin(), parseWarnings.end());
 
                 // if an arg parser was registered, but failed, bail
-                if (args == nullptr)
+                if (pfn && args == nullptr)
                 {
                     continue;
                 }