(a) Any person imprisoned by virtue of a sentence imposed by a state court of record who asserts that in the proceedings which resulted in his conviction there was a substantial denial of his rights under the Constitution of the United States or of this state may institute a proceeding under this article.

(b) The right to object to the composition of the grand or trial jury will be deemed waived under this Code section unless the person challenging the sentence shows in the petition and satisfies the court that cause exists for his being allowed to pursue the objection after the conviction and sentence have otherwise become final.