[TableGen][RISCV] Use getAllDerivedDefinitionsIfDefined in RISCVTargetDefEmitter (#91941)

getAllDerivedDefinitions produces a fatal error if there are no
definitions. In practice this isn't much of a problem for
llvm/lib/Target/RISCV/*.td where it's hard to imagine not having at
least one of the required defitions. But it limits our ability to
structure and maintain tests (which is how I came across this issue).

This commit moves to using getAllDerivedDefinitionsIfDefined and aims to
skip emission of data structures that make no sense if no definitions
were found.