[llvm-objcopy] --dump-section: error if '=' is missing or filename is empty

Fix PR45416: the diagnostic when '=' is missing is misleading.
`FileOutputBuffer::create` returns successfully when the filename is empty
(the temporary file is `.tmp%%%%%%%`), but `FileOutputBuffer::commit` will error when
renaming `.tmp%%%%%%%` to the empty name).

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D101697