Display the old color in ColorPicker for easier comparison (3.x)

[Calinou]

3.x version of #34840.

This only affects ColorPickerButton nodes that spawn a ColorPicker, not standalone ColorPickers.

Preview

[akien-mga]

The regression test project crashes:

ColorPickerButton._about_to_show --- executing with 0 parameters []
GDSCRIPT CODE:     ColorPickerButton.new()._about_to_show()
scene/gui/color_picker.cpp:913:23: runtime error: member call on null pointer of type 'struct ColorPicker'
scene/gui/color_picker.cpp:913:23: runtime error: member access within null pointer of type 'struct ColorPicker'
handle_crash: Program crashed with signal 11
Dumping the backtrace. Please include this when reporting the bug on godotengine/godot/issues
[1] bin/godot.x11.tools.64s() [0x173065a] (/home/runner/work/godot/godot/platform/x11/crash_handler_x11.cpp:54)
[2] /lib/x86_64-linux-gnu/libc.so.6(+0x46210) [0x7f589dc4a210] (??:0)
[3] ColorPickerButton::_about_to_show() (/home/runner/work/godot/godot/scene/gui/color_picker.cpp:913)
[4] MethodBind0::call(Object*, Variant const**, int, Variant::CallError&) (/home/runner/work/godot/godot/./core/method_bind.gen.inc:61 (discriminator 4))
...

[Calinou]

The regression test project crashes:

ColorPickerButton._about_to_show --- executing with 0 parameters []
GDSCRIPT CODE:     ColorPickerButton.new()._about_to_show()
scene/gui/color_picker.cpp:913:23: runtime error: member call on null pointer of type 'struct ColorPicker'
scene/gui/color_picker.cpp:913:23: runtime error: member access within null pointer of type 'struct ColorPicker'
handle_crash: Program crashed with signal 11
Dumping the backtrace. Please include this when reporting the bug on godotengine/godot/issues
[1] bin/godot.x11.tools.64s() [0x173065a] (/home/runner/work/godot/godot/platform/x11/crash_handler_x11.cpp:54)
[2] /lib/x86_64-linux-gnu/libc.so.6(+0x46210) [0x7f589dc4a210] (??:0)
[3] ColorPickerButton::_about_to_show() (/home/runner/work/godot/godot/scene/gui/color_picker.cpp:913)
[4] MethodBind0::call(Object*, Variant const**, int, Variant::CallError&) (/home/runner/work/godot/godot/./core/method_bind.gen.inc:61 (discriminator 4))
...

I added a null check, hopefully this should fix it. I'll also port this fix to master in another PR.

[akien-mga]

Thanks!