testharness.js: Fix logic in substitute_single when `subustitutions…

…` is undefined

The internal function `do_substitution(input)` was assuming that `substitutions` always exists or that the length of `components` would always be 1.

Fixes #46940.

resources/testharness.js

@@ -4408,13 +4408,20 @@
     {
         var substitution_re = /\$\{([^ }]*)\}/g;
 
-        function do_substitution(input) {
+        function do_substitution(input)
+        {
             var components = input.split(substitution_re);
             var rv = [];
-            for (var i = 0; i < components.length; i += 2) {
-                rv.push(components[i]);
-                if (components[i + 1]) {
-                    rv.push(String(substitutions[components[i + 1]]));
+            if (components.length === 1) {
+                rv = components;
+            } else if (substitutions) {
+                for (var i = 0; i < components.length; i += 2) {
+                    if (components[i]) {
+                        rv.push(components[i]);
+                    }
+                    if (substitutions[components[i + 1]]) {
+                        rv.push(String(substitutions[components[i + 1]]));
+                    }
                 }
             }
             return rv;