Fix empty XDG_CACHE_HOME Behavior

Use the default $HOME/.cache one for empty XDG_CACHE_HOME.

Fixes https://gitlab.freedesktop.org/fontconfig/fontconfig/-/issues/243

src/fccache.c

@@ -1412,6 +1412,8 @@ FcDirCacheWrite (FcCache *cache, FcConfig *config)
 	    }
 	}
     }
+    if (!test_dir)
+	fprintf (stderr, "Fontconfig error: No writable cache directories\n");
     if (d)
 	FcStrFree (d);
     FcStrListDone (list);

src/fccfg.c

@@ -2297,7 +2297,7 @@ FcConfigXdgCacheHome (void)
 
     if (!_FcConfigHomeEnabled)
 	return NULL;
-    if (env)
+    if (env && env[0])
 	ret = FcStrCopy ((const FcChar8 *)env);
     else
     {

test/run-test.sh

@@ -423,4 +423,34 @@ else
     echo "No test-crbug1004254: skipped"
 fi
 
+if [ "x$EXEEXT" = "x" ]; then
+
+dotest "empty XDG_CACHE_HOME"
+prep
+export XDG_CACHE_HOME=""
+export old_HOME="$HOME"
+export temp_HOME=$(mktemp -d --tmpdir fontconfig.XXXXXXXX)
+export HOME="$temp_HOME"
+cp "$FONT1" "$FONT2" "$FONTDIR"
+if [ -n "${SOURCE_DATE_EPOCH:-}" ] && [ ${#SOURCE_DATE_EPOCH} -gt 0 ]; then
+    touch -m -t "$(date -d @"${SOURCE_DATE_EPOCH}" +%y%m%d%H%M.%S)" "$FONTDIR"
+fi
+echo "<fontconfig><dir>$FONTDIR</dir><cachedir prefix=\"xdg\">fontconfig</cachedir></fontconfig>" > my-fonts.conf
+FONTCONFIG_FILE="$MyPWD"/my-fonts.conf $FCCACHE "$FONTDIR" || :
+if [ -d "$HOME"/.cache ] && [ -d "$HOME"/.cache/fontconfig ]; then : ; else
+  echo "*** Test failed: $TEST"
+  echo "No \$HOME/.cache/fontconfig directory"
+  ls -a "$HOME"
+  ls -a "$HOME"/.cache
+  exit 1
+fi
+
+export HOME="$old_HOME"
+rm -rf "$temp_HOME" my-fonts.conf
+unset XDG_CACHE_HOME
+unset old_HOME
+unset temp_HOME
+
+fi # if [ "x$EXEEXT" = "x" ]
+
 rm -rf "$FONTDIR" "$CACHEFILE" "$CACHEDIR" "$BASEDIR" "$FONTCONFIG_FILE" out