[DebugInfo] UnwindTable::create() should not add empty rows to CFI unwind table

UnwindTable::parseRows() may return successfully if the CFIProgram has either
no CFI instructions or only DW_CFA_nop instructions and the UnwindRow return
argument will be empty. But currently, the callers are not checking for this case
which is leading to incorrect dumps in the unwind tables in such cases i.e.

  CFA=unspecified

Reviewed By: clayborg

Differential Revision: https://reviews.llvm.org/D101892