DevTools: fix corner case of message formatter.

Bug: 833586
Change-Id: I87e956cadfa62a79aa549f22fe09211ee7cf25ce
Reviewed-on: https://chromium-review.googlesource.com/1015773Reviewed-by: Erik Luo <luoe@chromium.org>
Commit-Queue: Pavel Feldman <pfeldman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551532}

third_party/WebKit/LayoutTests/http/tests/devtools/unit/string-util-expected.txt

+
+test: testEmptyPrefixSuffix
+{
+    0 : {
+        precision : -1
+        specifier : "c"
+        substitutionIndex : 0
+        type : "specifier"
+    }
+    1 : {
+        precision : -1
+        specifier : "s"
+        substitutionIndex : 1
+        type : "specifier"
+    }
+}
+

third_party/WebKit/LayoutTests/http/tests/devtools/unit/string-util.js

+(async function() {
+    TestRunner.runTests([
+        function testEmptyPrefixSuffix()
+        {
+            const tokens = String.tokenizeFormatString(`%c%s`, {
+                c: () => {},
+                s: () => {}
+            });
+            TestRunner.addObject(tokens);
+        },
+    ]);
+})();

third_party/blink/renderer/devtools/front_end/platform/utilities.js

@@ -840,6 +840,8 @@ String.tokenizeFormatString = function(format, formatters) {
   let substitutionIndex = 0;
 
   function addStringToken(str) {
+    if (!str)
+      return;
     if (tokens.length && tokens[tokens.length - 1].type === 'string')
       tokens[tokens.length - 1].value += str;
     else