Allow NUL character in f-strings (#7378)

## Summary

This PR fixes the bug to allow `NUL` (`\0`) character inside f-strings.

## Test Plan

Add test case with `NUL` character inside f-string.

crates/ruff_python_parser/src/lexer.rs

@@ -566,7 +566,10 @@ impl<'source> Lexer<'source> {
 
         loop {
             match self.cursor.first() {
-                EOF_CHAR => {
+                // The condition is to differentiate between the `NUL` (`\0`) character
+                // in the source code and the one returned by `self.cursor.first()` when
+                // we reach the end of the source code.
+                EOF_CHAR if self.cursor.is_eof() => {
                     let error = if fstring.is_triple_quoted() {
                         FStringErrorType::UnterminatedTripleQuotedString
                     } else {
@@ -2079,6 +2082,12 @@ f"{(lambda x:{x})}"
         assert_debug_snapshot!(lex_source(source));
     }
 
+    #[test]
+    fn test_fstring_with_nul_char() {
+        let source = r"f'\0'";
+        assert_debug_snapshot!(lex_source(source));
+    }
+
     fn lex_fstring_error(source: &str) -> FStringErrorType {
         match lex(source, Mode::Module).find_map(std::result::Result::err) {
             Some(err) => match err.error {

crates/ruff_python_parser/src/snapshots/ruff_python_parser__lexer__tests__fstring_with_nul_char.snap

@@ -0,0 +1,25 @@
+---
+source: crates/ruff_python_parser/src/lexer.rs
+expression: lex_source(source)
+---
+[
+    (
+        FStringStart,
+        0..2,
+    ),
+    (
+        FStringMiddle {
+            value: "\\0",
+            is_raw: false,
+        },
+        2..4,
+    ),
+    (
+        FStringEnd,
+        4..5,
+    ),
+    (
+        Newline,
+        5..5,
+    ),
+]