Have the single octet known multiplier string types (IA5String,
NumericString, PrintableString, VisibleString, the yet unsupported
ISO646String) support returning the tvb containing the item in a
VAL_PTR if they don't have a permitted alphabet.

We currently handle a VAL_PTR of one of those types if they do
have a permitted alphabet.

Note that this also applies to UTCTime and GeneralizedTime, since
under the hood we treat those as VisibleStrings anyway.

It does *not* apply to BMPString (UCS2) nor UniversalSTring (UCS4),
even though those are known multiplier string types.

This should fix handling of ENBname and MMEname in S1AP as well.

Related to #19036