Add new base type handler for using nullable JDBC APIs

[kazuki43zoo]

I've fixed #1242 by another approach(adding an extension point on BaseTypeHandler and adding extension class).

@harawata @mnesarco Which is better?

[harawata]

I guess you are trying to improve other type handlers.
I think it's a good idea, but the implementation looks overly complicated for its purpose.

We should simply remove wasNull() call from the base type handler, probably [1].

public abstract class BaseTypeHandler<T> extends TypeReference<T> implements TypeHandler<T> {

  // setParameter() is as-is

  @Override
  public T getResult(ResultSet rs, String columnName) throws SQLException {
    T result;
    try {
      result = getNullableResult(rs, columnName);
    } catch (Exception e) {
      throw new ResultMapException("Error attempting to get column '" + columnName + "' from result set.  Cause: " + e, e);
    }
  }

  // pretty much the same
}

Most type handler implementations may stay unchanged and those who need wasNull() can check the result before returning the result.
Here are a few examples.

public class IntegerTypeHandler extends BaseTypeHandler<Integer> {

  // setNonNullParameter() is as-is

  @Override
  public Integer getNullableResult(ResultSet rs, String columnName)
      throws SQLException {
    int result = rs.getInt(columnName);
    return result == 0 && rs.wasNull() ? null : Integer.valueOf(result);
  }
  
  // pretty much the same
}

public class BooleanTypeHandler extends BaseTypeHandler<Boolean> {

  // setNonNullParameter() is as-is

  @Override
  public Boolean getNullableResult(ResultSet rs, String columnName)
      throws SQLException {
    boolean result = rs.getBoolean(columnName);
    return result ? Boolean.TRUE : (rs.wasNull() ? null : Boolean.FALSE);
  }

  // same
}

We don't need the extra type handler anymore and it has better reusability (e.g. subclasses can return non-null value when the column is NULL).

[1] I think it's OK to change the existing BaseTypeHandler even though it might surprise a few users. Alternatively, we can add a new base type handler and use it as a parent of all internal type handlers, but I don't like this idea very much.

[kazuki43zoo]

@harawata OK. I agree with your opinion. I will submit new PR at later.