Any agreement, understanding or practice, written or oral, implied or expressed, between any labor organization and employer which violates the rights of employees as guaranteed by provisions of this chapter is hereby declared to be unlawful, null and void, and of no legal effect. Any strike, picketing, boycott, or other action by a labor organization for the sole purpose of inducing or attempting to induce an employer to enter into any agreement prohibited under this chapter is hereby declared to be for an illegal purpose and is a violation of the provisions of this chapter.