(a) If evidence concerning execution of an attested will that is not self-proved is necessary in contested cases, the testimony of at least one of the attesting witnesses, if within the state, competent, and able to testify, is required. Due execution of an attested or unattested will may be proved by other evidence.
(b) If the will is self-proved, compliance with signature requirements for execution is conclusively presumed 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 to it, unless there is proof of fraud or forgery affecting the acknowledgment or affidavit.