tests: use 'test_must_be_empty' instead of 'test ! -s'

Using 'test_must_be_empty' is preferable to 'test ! -s', because it
gives a helpful error message if the given file is unexpectedly no
empty, while the latter remains completely silent.  Furthermore, it
also catches cases when the given file unexpectedly does not exist at
all.

This patch was created by:

  sed -i -e 's/test ! -s/test_must_be_empty/' t[0-9]*.sh

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4201-shortlog.sh

@@ -192,7 +192,7 @@ test_expect_success 'shortlog with revision pseudo options' '
 test_expect_success 'shortlog with --output=<file>' '
 	git shortlog --output=shortlog -1 master >output &&
-	test ! -s output &&
+	test_must_be_empty output &&
 	test_line_count = 3 shortlog
 '

t/t4211-line-log.sh

@@ -102,7 +102,7 @@ test_expect_success '-L with --first-parent and a merge' '
 test_expect_success '-L with --output' '
 	git checkout parallel-change &&
 	git log --output=log -L :main:b.c >output &&
-	test ! -s output &&
+	test_must_be_empty output &&
 	test_line_count = 70 log
 '

t/t8010-cat-file-filters.sh

@@ -47,7 +47,7 @@ test_expect_success 'cat-file --textconv --path=<path> works' '
 test_expect_success '--path=<path> complains without --textconv/--filters' '
 	sha1=$(git rev-parse -q --verify HEAD:world.txt) &&
 	test_must_fail git cat-file --path=hello.txt blob $sha1 >actual 2>err &&
-	test ! -s actual &&
+	test_must_be_empty actual &&
 	grep "path.*needs.*filters" err
 '

t/t9802-git-p4-filetype.sh

@@ -310,7 +310,7 @@ test_expect_success SYMLINKS 'empty symlink target' '
 		# p4 to sync here will make it generate errors.
 		cd "$cli" &&
 		p4 print -q //depot/empty-symlink#2 >out &&
-		test ! -s out
+		test_must_be_empty out
 	) &&
 	test_when_finished cleanup_git &&