fix(challenge modal): user could submit empty answer

src/components/challenge-modal/challenge-modal.tsx

@@ -45,6 +45,7 @@ const Challenge = ({ challenge, closeModal }: ChallengeProps) => {
 
   const onEnterKey = (e: React.KeyboardEvent<HTMLInputElement>) => {
     if (e.key !== 'Enter') return;
+    if (!answers[currentChallengeIndex]) return;
     if (challenges[currentChallengeIndex + 1]) {
       setCurrentChallengeIndex((prev) => prev + 1);
     } else {
@@ -112,7 +113,11 @@ const Challenge = ({ challenge, closeModal }: ChallengeProps) => {
         <div className={styles.challengeFooter}>
           <div className={styles.counter}>{t('challenge_counter', { index: currentChallengeIndex + 1, total: challenges?.length })}</div>
           <span className={styles.buttons}>
-            {!challenges[currentChallengeIndex + 1] && <button onClick={onSubmit}>{t('submit')}</button>}
+            {!challenges[currentChallengeIndex + 1] && (
+              <button onClick={onSubmit} disabled={!answers[currentChallengeIndex]}>
+                {t('submit')}
+              </button>
+            )}
             {challenges.length > 1 && (
               <button disabled={!challenges[currentChallengeIndex - 1]} onClick={() => setCurrentChallengeIndex((prev) => prev - 1)}>
                 {t('previous')}