astextplain: restore passthrough for empty files

Our astextplain helper used to pass through empty files and plaintext with the
an expected extension. WHen we changed it to work based on mimetype
rather than a predefined list of extensions in git-for-windows#624, both those passthroughs got
lost. Plaintext passthrough has been restored in git-for-windows#626, but passthrough
of empty files is still missing. Restore this functionality.

This fixes git-for-windows/git#5977

Signed-off-by: Matthias Aßhauer <mha1993@live.de>

Author: rimrul

git-extra/PKGBUILD

@@ -81,7 +81,7 @@ sha256sums=('8ed76d1cb069ac8568f21c431f5e23caebea502d932ab4cdff71396f4f0d5b72'
             'dcbd1b981d5b899afe30151a5f5a27ea52025ff1335f131af1b5891f62ddd55b'
             '683ab066be19cb4defec470ebd53f165ca5dbf761fd40c13aee8abe31ba42803'
             '1cf2c13fb97c51375a76ed479362c3cbcdb51ab4d3a745e8d2c3a780badd8d46'
-            '32223da4e1264d0c9663dd3d4b347131a634e96c2676d673ca58b2a6c58d2aea'
+            'c6a92fc3bc23f19d396e9544353495d4591be481e9890f7bbb5968f7693091da'
             '1fae4ff3ce1e75ac90fe6f6130db4d041bc34f59deea679501f57d2c41d37560'
             '22f41610dea842890955032af30efdb60e80f310e95a04e57ab36b10e0376923'
             '8120b41b869693e07acc4707a0ab45505e7c5e4156a3a44b19152f1f425df85f'

git-extra/astextplain

@@ -21,7 +21,7 @@ case "$(file --brief --mime-type "$1")" in
 		out=$(pdftotext -q -layout -enc UTF-8 "$1" -) && sed "s/(\^M$)|(^\^L)//" <<<$out || cat "$1"
 		;;
 	# TODO add rtf support
-	application/rtf | text/rtf | text/plain)
+	application/rtf | text/rtf | text/plain | inode/x-empty)
 		cat "$1"
 		;;
 	*)