[devtools] Defend against source locations without associated script

Apparently, we can get raw locations with a `null` content provider.
This CL filters them until we can prevent their generation altogether.

Bug: chromium:1005789
Change-Id: I02ababe8c3f3e355e9bc48622f15aa7c0d5f18b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1819300Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698889}

third_party/blink/renderer/devtools/front_end/sources/SourceFormatter.js

@@ -223,7 +223,7 @@ Sources.SourceFormatter.ScriptMapping = class {
     }
     if (uiSourceCode.contentType().isScript()) {
       const rawLocations = Bindings.debuggerWorkspaceBinding.uiLocationToRawLocations(uiSourceCode, 0, 0);
-      return rawLocations.map(location => location.script());
+      return rawLocations.map(location => location.script()).filter(script => !!script);
     }
     return [];
   }