refactor: replace nil checks with not in conditions

[omarcopires]

Pull Request Prelude

• I have followed proper The Forgotten Server code styling.
• I have read and understood the contribution guidelines before making this PR.
• I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

Changes Proposed

This commit introduces a refactor to replace nil checks with the use of not in logical conditions, aligning with the preferred style in the codebase. This change improves code readability and consistency throughout the affected sections.

[Zbizu]

this change introduces nil comparisons which tfs moved away from, except for situations when both nil and false can be returned by a function and need to be distinguished

if the statement does not need to distinguish between false and nil:

• ~= nil is not necessary in logical statements
• if x == nil then should be replaced with if not x then

by the way, can storage checks return nil now?

[MillhioreBT]

What @Zbizu says is true, please make the change from if not x then
and for cases of setStorageValue(-1), change to removeStorageValue

[luanluciano93]

Every time I pull a value from storage, will I have to check to see if it is not null? There is no logic in changing this.

[MillhioreBT]

Change what you check on line 16 in login.lua