What is the difference between "an" (أنْ) and "anna" (أنّ) in the testimony "لا إله إلا الله", and when is each one used?

The nun in "an" (أنْ) in "Ashhadu an la ilaha illa Allah" (I bear witness that there is no god but Allah) is silent (sukūn) and assimilated into the lam. Its origin is "anna" (أنَّ) in its lightened form, where its subject (ismuha) is a hidden pronoun of the "affair" or "situation" (ḍamīr ash-sha'n), and its predicate (khabaruha) is the phrase "la ilaha" (there is no god). Ibn Malik said: "If 'anna' is lightened, its subject is hidden, and make the predicate a جمله ( جمله) after 'an'." It is not permissible to pronounce it with a shaddah (doubled) here due to the absence of its subject, unlike "Ashhadu anna Muhammadan Rasoolullah" (I bear witness that Muhammad is the Messenger of Allah), where the nun is doubled and the subject (Muhammadan) is mentioned.