[BOLT] Fix empty function emission in non-relocation mode

In non-relocation mode, every function is emitted in its own section. If
a function is empty, RuntimeDyld will still allocate 1-byte section
for the function and initialize it with zero. As a result, we will
overwrite the first byte of the original function contents with zero.
Such scenario can happen when the input function had only NOP
instructions which BOLT removes by default. Even though such functions
likely cause undefined behavior, it's better to preserve their contents.

Reviewed By: yota9

Differential Revision: https://reviews.llvm.org/D133978