[jOOQ/jOOQ#9557] Add support for ALTER TABLE .. DROP CONSTRAINT IF

EXISTS

- Added DDL interpreter support

jOOQ/src/main/java/org/jooq/impl/DDLInterpreter.java

@@ -500,7 +500,8 @@ final class DDLInterpreter {
                     }
                 }
 
-                throw constraintNotExists(query.$dropConstraint());
+                if (!query.$ifExistsConstraint())
+                    throw constraintNotExists(query.$dropConstraint());
             }
         }
         else if (query.$dropConstraintType() == PRIMARY_KEY) {