Fix error when enabling texture packs in the "Content" tab

[grorp]

Since #13550, enabling or disabling texture packs in the "Content" tab triggers this error:

Runtime error from mod '??' in callback handleMainMenuButtons(): /mt/bin/../builtin/common/filterlist.lua:209: attempt to index local 'self' (a nil value)
stack traceback:
	/mt/bin/../builtin/common/filterlist.lua:209: in function 'size'
	/mt/bin/../builtin/mainmenu/tab_content.lua:69: in function 'get_formspec'
	/mt/bin/../builtin/fstk/tabview.lua:67: in function 'get_formspec'
	/mt/bin/../builtin/fstk/ui.lua:101: in function 'update'
	/mt/bin/../builtin/fstk/ui.lua:145: in function 'handle_buttons'
	/mt/bin/../builtin/fstk/ui.lua:186: in function </mt/bin/../builtin/fstk/ui.lua:172>

This PR fixes that by restoring the previous behavior of re-creating the filterlist in get_formspec if it is nil.

CC @rollerozxa

To do

This PR is a Ready for Review.

How to test

Verify that you can enable and disable texture packs without triggering an error dialog.

[SmallJoker]

Error reproduced, fix reproduced. Works.