LibWeb: Resolve padding against 0 and not inf for indefinite width more

[Lubrsi]

In particular, in BFC:

• Non-floating, non-replaced elements
• Floating, non-replaced elements
• Floating, replaced elements

The first two regressed in https://github.com/SerenityOS/serenity/commit/1d76126abe9dd65251bd28a381350f70083b50f7

The third one seems to have been introduced by this regression, as it was seemingly copied from compute_width_for_floating_box in https://github.com/SerenityOS/serenity/commit/7f9ede07bcf287a2b16ff29c1ebbcf64c47a221f