Validate empty regex password policy with white-space characters (#22807)

Closes #20411

Signed-off-by: Pedro Hos <pedro-hos@outlook.com>

server-spi-private/src/main/java/org/keycloak/policy/RegexPatternsPasswordPolicyProvider.java

@@ -57,6 +57,8 @@ public class RegexPatternsPasswordPolicyProvider implements PasswordPolicyProvid
     public Object parseConfig(String value) {
         if (value == null) {
             throw new PasswordPolicyConfigException("Config required");
+        } else if(value.trim().isEmpty()) {
+            throw new PasswordPolicyConfigException("White space not acepted");
         }
         try {
             return Pattern.compile(value);