This commit updates the `submit_button()` function to standardize the default values of `$text` and `$other_attributes` parameters. Previously set to `null`, these defaults have now been changed to an empty string (`''`), bringing consistency with the `get_submit_button()` function, which `submit_button()` wraps.
The change maintains backward compatibility, as `get_submit_button()` does not perform strict type-checking on these parameters. This update aligns with the ongoing effort to standardize function parameters across the WordPress codebase, improving the readability and predictability of the core functions.
Follow-up to [31446].
Props andbalashov.
Fixes#59921.
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