The DWARF standard says that an empty compile unit is not valid:
Each such contribution consists of a compilation unit header (see Section 7.5.1.1 on page 200) followed by a single DW_TAG_compile_unit or DW_TAG_partial_unit debugging information entry, together with its children.
Therefore we shouldn't clone them in dsymutil.
This sentence seems incomplete ("Dsymutil should not generate an empty CU and it", "and it" what?)