Fix minor shellcheck 0.7.2 warnings

The first warning of a misspelling is a false positive, so we annotate
the script accordingly.  As for the x-prefix warnings update the check
to use the conventional '[ -z <string> ]' syntax.

all-syslog.sh:46:47: warning: Possible misspelling: ZEVENT_ZIO_OBJECT
    may not be assigned, but ZEVENT_ZIO_OBJSET is. [SC2153]
make_gitrev.sh:53:6: note: Avoid x-prefix in comparisons as it no
    longer serves a purpose [SC2268]
man-dates.sh:10:7: note: Avoid x-prefix in comparisons as it no
    longer serves a purpose [SC2268]

Reviewed-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #12208

scripts/make_gitrev.sh

@@ -50,7 +50,7 @@ esac
 ZFS_GITREV=$({ cd "${top_srcdir}" &&
 	git describe --always --long --dirty 2>/dev/null; } || :)
 
-if [ "x${ZFS_GITREV}" = x ]
+if [ -z "${ZFS_GITREV}" ]
 then
 	# If the source directory is not a git repository, check if the file
 	# already exists (in the source)

scripts/man-dates.sh

@@ -7,6 +7,6 @@ set -eu
 
 find man -type f | while read -r i ; do
     git_date=$(git log -1 --date=short --format="%ad" -- "$i")
-    [ "x$git_date" = "x" ] && continue
+    [ -z "$git_date" ] && continue
     sed -i "s|^\.Dd.*|.Dd $(date -d "$git_date" "+%B %-d, %Y")|" "$i"
 done