Fix metadata storing empty dictionary in scenes

[Xrayez]

Fixes an issue described in #22642 (comment).

Completely not sure whether it's a good way to manipulate property usage flags this way. There was some flags that allowed this kind of usage but those are deprecated now (which might explain why it worked a year ago @YeldhamDev):

godot/core/object.h

Lines 107 to 109 in 2ca3e47

	//those below are deprecated thanks to ClassDB's now class value cache
	//PROPERTY_USAGE_STORE_IF_NONZERO = 512, //only store if nonzero
	//PROPERTY_USAGE_STORE_IF_NONONE = 1024, //only store if false

This would prevent unnecessary noise in version control systems for projects already in work.

[Xrayez]

This makes it PROPERTY_USAGE_DEFAULT.

[akien-mga]

It seems to work fine for script without the need to do this conditional storage, I think the issue is deeper than that (likely related to the Dictionary type). That's why in #22642 (comment) it also appears as "revertable" even when empty.

[akien-mga]

So far there's no other exposed property of type Dictionary which is not using either PROPERTY_USAGE_INTERNAL and/or PROPERTY_USAGE_NOEDITOR.

I guess the logic for EditorPropertyRevert::can_property_revert() doesn't properly handle Dictionary, and maybe the logic used for export (Dictionary) is different? (CC @bojidar-bg)

[bojidar-bg]

I think #29222 would resolve this. Currently Dictionary equality checks are done by reference, unlike other non-Object types, which are compared by value.

[Xrayez]

Yes, #29222 does solve this, manually cherry-picked it locally on top of master to test it, so this one could be closed once that's merged. It also get rids of the issue with appearing "revertable".

[akien-mga]

Superseded by #30749 (and #29222 once it would be merged).