Fix: Allow nullptr callback in evaluateJavascript on Linux

The callback parameter of WebBrowserComponent::evaluateJavascript has
a default value of nullptr. Updated the Linux implementation to handle
this case properly by checking if the callback queue is empty before
attempting to invoke the callback.

modules/juce_gui_extra/native/juce_WebBrowserComponent_linux.cpp

@@ -1308,12 +1308,18 @@ public:
     {
         const auto params = FromVar::convert<EvaluateJavascriptCallbackParams> (paramsIn);
 
-        if (! params.has_value() || evaluationCallbacks.size() == 0)
+        if (! params.has_value())
         {
             jassertfalse;
             return;
         }
 
+        // do nothing if there are no evaluation callbacks
+        if (evaluationCallbacks.empty()) 
+        {
+            return;
+        }
+
         const auto result = [&]
         {
             using Error = EvaluationResult::Error;