StatementUtil的getTypeOfNull函数，在某些情况下会空指针，需要规避

[yisiliang]

某些数据库的getParameterMetaData 会返回 NULL，然后导致报错，也需要规避这种情况
原代码：

	public static int getTypeOfNull(PreparedStatement ps, int paramIndex) {
		int sqlType = Types.VARCHAR;

		final ParameterMetaData pmd;
		try {
			pmd = ps.getParameterMetaData();
			sqlType = pmd.getParameterType(paramIndex);
		} catch (SQLException ignore) {
			// ignore
			// log.warn("Null param of index [{}] type get failed, by: {}", paramIndex, e.getMessage());
		}

		return sqlType;
	}

需改成

	public static int getTypeOfNull(PreparedStatement ps, int paramIndex) {
		int sqlType = Types.VARCHAR;

		final ParameterMetaData pmd;
		try {
			pmd = ps.getParameterMetaData();
			if (pmd == null) {
			    return sqlType;
			}
			sqlType = pmd.getParameterType(paramIndex);
		} catch (SQLException ignore) {
			// ignore
			// log.warn("Null param of index [{}] type get failed, by: {}", paramIndex, e.getMessage());
		}

		return sqlType;
	}

[looly]

PR已接受，感谢。