rust: list: Use "List::is_empty()" to perform checking when possible

rust: list: Use "List::is_empty()" to perform checking when possible

"List::is_empty()" provides a straight forward convention to check
whether a given "List" is empty or not. There're numerous places in the
current implementation still use "self.first.is_null()" to perform the
equivalent check, replace them with "List::is_empty()".

Signed-off-by: I Hsin Cheng <richard120310@gmail.com>
Link: https://lore.kernel.org/r/20250310073853.427954-1-richard120310@gmail.com
Reviewed-by: Benno Lossin <lossin@kernel.org>
[ Rebased dropping the cases that do not apply anymore. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>