It is valid for an Allergies and Intolerances section entry to contain only the text name of a drug (like penicillin) without an RxNorm or SNOMED CT code. However, the specification is a bit ambiguous on whether that is legal. Volume 1 Section 3.6 states as follows.

Any SHALL, SHOULD or MAY conformance statement may use nullFlavor, unless the nullFlavor is explicitly disallowed (e.g., through another conformance statement which includes a SHALL conformance for a vocabulary binding to the @code attribute, or through an explicit SHALL NOT allow use of nullFlavor conformance).

Volume 2 section 3.105.1 contains this conformance statement for the Manufactured Material participant.

This playingEntity SHALL contain exactly one [1..1] code, which SHALL be selected from ValueSet Substance Reactant for Intolerance 2.16.840.1.113762.1.4.1010.1 DYNAMIC (CONF:1098-7419).

I think some implementers could be confused by this because although there is a SHALL conformance for a vocabulary binding, the binding is to the code element rather than the @code attribute. I recommend we update CONF:1098-7419 with an explicit statement that a @nullFlavor attribute is permitted. Or alternatively, we could add a more general clarification to Volume 1 Section 3.6 stating that a SHALL vocabulary binding to a whole code element, and not specifically to the contained @code attribute, does not implicitly disallow the use of a @nullFlavor attribute.