Allow for the case where nsIScriptError objects have no sourceName.
Allow for the case where nsIScriptError objects have no sourceName.

atlassian/JSErrorCollector,chrisdarroch/JSErrorCollector,atlassian/JSErrorCollector,atlassian/JSErrorCollector,atlassian/JSErrorCollector,chrisdarroch/JSErrorCollector,chrisdarroch/JSErrorCollector,chrisdarroch/JSErrorCollector