[MC][RISCV] Check hasEmitNops before call shouldInsertExtraNopBytesForCodeAlign (#77236)

The shouldInsertExtraNopBytesForCodeAlign() need STI to check whether
relax is enabled or not. It is initialized when call setEmitNops. The
setEmitNops may not be called in a section which has instructions but is
not executable. In this case uninitialized STI will cause problems.
Thus, check hasEmitNops before call it.

Fixes:
https://github.com/llvm/llvm-project/pull/76552#issuecomment-1878952480