expr: fix tikv crash when conv empty string (#12672) (#12692)

ref #12672, close #12673

Fix tikv crash when conv empty string

Signed-off-by: ti-srebot <ti-srebot@pingcap.com>

Co-authored-by: Shenghui Wu <793703860@qq.com>
Co-authored-by: Ti Chi Robot <ti-community-prow-bot@tidb.io>

components/tidb_query_expr/src/impl_math.rs

@@ -612,7 +612,7 @@ fn is_valid_base(base: IntWithSign) -> bool {
 
 fn extract_num_str(s: &str, from_base: IntWithSign) -> Option<(String, bool)> {
     let mut iter = s.chars().peekable();
-    let head = *iter.peek().unwrap();
+    let head = *iter.peek()?;
     let mut is_neg = false;
     if head == '+' || head == '-' {
         is_neg = head == '-';
@@ -1573,6 +1573,7 @@ mod tests {
             ("16九a", 10, 8, "20"),
             ("+", 10, 8, "0"),
             ("-", 10, 8, "0"),
+            ("", 2, 16, "0"),
         ];
         for (n, f, t, e) in tests {
             let n = Some(n.as_bytes().to_vec());