[RISCV] Fix required features checking with empty string

In our downstream, we define some intrinsics that don't require any
extra extension enabled. Such as

TARGET_BUILTIN(__builtin_riscv_xxx, "LiLi", "nc", "")

But `split` function's `KeepEmpty` argument is True. Got the error message

error: builtin requires at least one of the following extensions support to be enabled : ''

when we use our customized intrinsic.

Reviewed By: craig.topper, wangpc

Differential Revision: https://reviews.llvm.org/D154596