gpg: fix --enarmor with zero length source file

* common/iobuf.c (filter_flush): Remove "src_len == 0" check.
* g10/compress-bz2.c (do_compress): Exit early if flush not
forced and input length is zero.
* g10/compress.c (do_compress): Likewise.

Remove "(src_len == 0)" check in filter_flush which was
introduced to fix compress failure caused by zero length
flush from iobuf_close. However this check broke enarmoring
file with length of zero. Patch instead fixes zero length
flush problem in compress filters.

• GnuPG-bug-id: T5828
• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>