makepkg: ignore empty global attributes in extraction

This bug isn't currently exposed by any of the existing codepaths, but
an upcoming patch to introduce SRCINFO files to makepkg will expose
this.

Signed-off-by: Allan McRae <allan@archlinux.org>

scripts/makepkg.sh.in

@@ -2385,7 +2385,7 @@ extract_global_var() {
 		array_build ref "$attr"
 		[[ ${ref[@]} ]] && array_build "$outputvar" "$attr"
 	else
-		[[ -v $attr ]] && printf -v "$outputvar" %s "${!attr}"
+		[[ ${!attr} ]] && printf -v "$outputvar" %s "${!attr}"
 	fi
 }