rust: add additional tests to test_trim_whitespace

Signed-off-by: Bennet Bleßmann <[email protected]>

rust/kernel/str.rs

@@ -602,6 +602,8 @@ macro_rules! fmt {
 ///
 /// ```
 /// # use kernel::str::trim_whitespace;
+/// assert_eq!(trim_whitespace(b""       ), b"");
+/// assert_eq!(trim_whitespace(b"       "), b"");
 /// assert_eq!(trim_whitespace(b"foo    "), b"foo");
 /// assert_eq!(trim_whitespace(b"    foo"), b"foo");
 /// assert_eq!(trim_whitespace(b"  foo  "), b"foo");