ApplicationLanguages.PrimaryLanguageOverride cannot be unset

### Describe the bug

The Microsoft.Windows.Globalization.ApplicationLanguages.PrimaryLanguageOverride property by default returns an empty string (if the app has not set a language override before). However, once a language override was set, it can no longer be unset to remove the override. Setting `PrimaryLanguageOverride = ""` or `PrimaryLanguageOverride = null` will both throw an exception.

In comparison, Windows.Globalization.ApplicationLanguages.PrimaryLanguageOverride _does_ allow setting an empty string, which will clear the previous override. However setting Windows.Globalization.ApplicationLanguages.PrimaryLanguageOverride will not clear the Microsoft.Windows.Globalization.ApplicationLanguages.PrimaryLanguageOverride property for packaged apps, so it is only an incomplete workaround.

The following check must be changed to also allow an empty string: https://github.com/microsoft/WindowsAppSDK/blob/3d8013bbcaaa1cc9da3fcdf0d453d0017027493e/dev/MRTCore/mrt/Microsoft.Windows.ApplicationModel.Resources/src/ApplicationLanguages.cpp#L44

### Steps to reproduce the bug

1. Set the override to some language, e.g. `Microsoft.Windows.Globalization.ApplicationLanguages.PrimaryLanguageOverride = "fr-FR";`
2. Try to unset the override, e.g. `Microsoft.Windows.Globalization.ApplicationLanguages.PrimaryLanguageOverride = "";`

### Expected behavior

No exception is thrown when an empty string is set, and the language override will be reset.

### Screenshots

_No response_

### NuGet package version

Windows App SDK 1.7.1: 1.7.250401001

### Packaging type

Packaged (MSIX)

### Windows version

Windows 11 version 24H2 (22621, October 2024 Update)

### IDE

Visual Studio 2022

### Additional context

_No response_

Inspect dev/MRTCore/mrt/Microsoft.Windows.ApplicationModel.Resources/src/ApplicationLanguages.cpp at the check around line 44. Reproduce the issue by setting PrimaryLanguageOverride to "fr-FR" and then to an empty string, and compare the behavior with Windows.Globalization.ApplicationLanguages.PrimaryLanguageOverride. Done means an empty string no longer throws and resets the language override.

Written by the indexing model from the issue text.