fix: validationUtil

Author: QuentinTournier40

src/main/java/dev/resms/util/ValidationUtil.java

@@ -12,10 +12,14 @@ public class ValidationUtil {
      * @throws ReSMSException if the phone number is invalid
      */
     public static void validatePhoneNumber(String phoneNumber) throws ReSMSException {
-        if (phoneNumber == null || phoneNumber.trim().isEmpty()) {
+        if (phoneNumber == null) {
             throw new ReSMSException("Phone number is required");
         }
 
+        if (phoneNumber.trim().isEmpty()) {
+            throw new ReSMSException("Phone number cannot be empty");
+        }
+
         String cleanNumber = phoneNumber.trim();
         if (!cleanNumber.matches("^\\+?[1-9]\\d{1,14}$")) {
             throw new ReSMSException("Invalid phone number format");

src/test/java/dev/resms/util/ValidationUtilTest.java

@@ -16,7 +16,7 @@ void testValidatePhoneNumber_validNumber() {
     @Test
     void testValidatePhoneNumber_emptyNumber() {
         ReSMSException ex = assertThrows(ReSMSException.class, () -> ValidationUtil.validatePhoneNumber("  "));
-        assertEquals("Phone number is required", ex.getMessage());
+        assertEquals("Phone number cannot be empty", ex.getMessage());
     }
 
     @Test