Animation lost when opening projects created in 4.3

[timothyqiu]

Tested versions

• Reproducible: master[e2dd56b]
• Not reproducible: 4.4.dev1

System information

Arch Linux

Issue description

When opening a project created in 4.3, animations in AnimationPlayers are lost with this error:

ERROR: Condition "p_animation_library.is_null()" is true. Returning: ERR_INVALID_PARAMETER
   at: add_animation_library (scene/animation/animation_mixer.cpp:281)

Probably an regression from #70096

Steps to reproduce

• Create a project in 4.3, add an AnimationPlayer in the scene and create a new Animation in it.
• Open the project with an editor built from master.

Minimal reproduction project (MRP)

test.zip

[akien-mga]

Probably an regression from #70096

Confirmed, a local revert of that PR solves the issue (on top of 906a4e9).

CC @rune-scape @godotengine/core @godotengine/animation

[rune-scape]

the underlying issue is that the hash for an empty String is different than the hash for an empty StringName
and the default animation library is an empty string
so when the String key is converted to a StringName, it hashes differently
making a PR