fix(macos/dialog): avoid setting empty default_path

[amrbashir]

closes #7566

ref: #4065
ref: #4028
ref: #9762

[FabianLars]

Are strings/paths containing only whitespace a concern too? (Not as common as empty paths ig but worth to double check while we're at it maybe)

[amrbashir]

I am reluctant to adding this fix and lean towards making it the developer responsibility to pass defaultPath: null instead of empty but seeing that some tauri users fall in this crash quite often, we should add this fix but other than this, it shouldn't be our problem and they should handle it themselves.

[FabianLars]

ok