fix(DatetimePicker): infinite loop when use formatted text is unnumeric (#4485)

src/datetime-picker/utils.ts

@@ -17,7 +17,11 @@ export function getTrueValue(value: string | undefined): number | undefined {
   }
 
   while (isNaN(parseInt(value, 10))) {
-    value = value.slice(1);
+    if (value.length > 1) {
+      value = value.slice(1);
+    } else {
+      return;
+    }
   }
 
   return parseInt(value, 10);