do not return virtual file system path as default path in the file content widget (refs #37308)

[alexbruy]

Description

North arrow layout item uses :/images/north_arrows/layout_default_north_arrow.svg as a default image. When user tries to change image by clicking on "…" button the file dialog will use this path as a working directory (if this is a first time action and last used path is not saved in the QGIS settings) and as this is not an actual path on the filesystem, user will see a warning message "The file or folder does not exists".

We should not use virtual file system paths as a default path for file dialogs.

Refs #37308.

AI tool usage

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[alexbruy]

I also removed duplicated call to setLastPathSettingsKey() in the QgsLayoutPictureWidget introduced in #43119, using the value which is set last.