safe_file_write() doesn't like null for its $backup_file argument

[Sesquipedalian]

Description

I don't recall seeing this in earlier PHP versions, but at least in PHP 8.2, if the $backup_file argument for safe_file_write() is null, errors will be generated on line 1874 about file_exists() not liking null arguments, and then on line 1929 about $temp_bfile not being defined.

Steps to reproduce

1. Write some code that calls safe_file_write($file, $data) (note the lack of a $backup_file argument).
2. Run that code.

Environment (complete as necessary)

• Version/Git revision: 2.1.4
• Database Type: N/A
• Database Version: N/A
• PHP Version: 8.2.5

Additional information/references

SMF itself doesn't encounter this bug because we always call safe_file_write() with the $backup_file argument set to a file path. But I ran into it while testing updates to a mod that calls the function without a $backup_file argument.