(a) If evidence concerning execution of an attested will which is not self-proved is necessary in contested cases, the testimony of at least one of the attesting witnesses is required. Such testimony is not required if: (1) no attesting witness is within the State; (2) no attesting witness is competent to testify; (3) no attesting witness can be found; or (4) all attesting witnesses are otherwise unable to testify. Due execution of an attested will may be proved by other evidence.
(b) If the will is self-proved, compliance with signature requirements for execution and other requirements of execution are presumed subject to rebuttal, without the testimony of any witness upon filing the will and the acknowledgment and affidavits annexed or attached thereto, unless there is proof of fraud or forgery affecting the acknowledgment or affidavit.