Fix "x" in shellcheck

As seen here, the newest version of shellcheck flags this pattern of
if [ "x" == "x$1" ]
as no longer needed.

https://jenkins.openbmc.org/job/CI-MISC/job/ci-openbmc-build-scripts/456/console

Interestingly, this actually causes openbmc-build-scripts to fail as a
project, which I don't fully understand why, but this seems unimportant
to the low effort, low impact fix to that it required.

Tested: I have no idea what this script does or how to test it, based on
the name I assume CI testing is enough, but changes were suggested by
the robot, and seem trivial.

Related change to scripts within openbmc-build-scripts:
https://gerrit.openbmc-project.xyz/c/openbmc/openbmc-build-scripts/+/51279

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: Ibaa1dbf788a0dbc4be28cf124b6677fb8cb9b99b
diff --git a/tools/sdbus++-gen-meson b/tools/sdbus++-gen-meson
index e2da6eb..8a3ddf3 100755
--- a/tools/sdbus++-gen-meson
+++ b/tools/sdbus++-gen-meson
@@ -149,7 +149,7 @@
         meson_path="$meson_path/$part"
 
         # Create the meson.build for this segment if it doesn't already exist.
-        if [ "x" == "x${meson_paths[$meson_path]}" ];
+        if [ "" == "${meson_paths[$meson_path]}" ];
         then
             meson_paths["$meson_path"]="1"
             meson_empty_file "$meson_path"
@@ -158,7 +158,7 @@
             # We need to skip adding the links into the 'root' meson.build
             # because most repositories want to selectively add TLDs based
             # on config flags.  Let them figure out their own logic for that.
-            if [ "x$outputdir" != "x$prev_meson_path" ];
+            if [ "$outputdir" != "$prev_meson_path" ];
             then
                 echo "subdir('$part')" >> "$prev_meson_path/meson.build"
             fi
@@ -295,7 +295,7 @@
 ## For an interface foo/bar, the outputdir is expected to be foo/bar.
 function cmd_cpp {
 
-    if [ "x" == "x$1" ];
+    if [ "" == "$1" ];
     then
         show_usage
         exit 1
@@ -333,7 +333,7 @@
 ## For an interface foo/bar, the outputdir is expected to be foo.
 function cmd_markdown {
 
-    if [ "x" == "x$1" ];
+    if [ "" == "$1" ];
     then
         show_usage
         exit 1