AudioProcessorGraph: Fix MIDI channel bug and graph execution modification

MIDI only plugins are no longer provided valid audio buffers
Graph nodes are only executed when they have active connections

examples/Plugins/ArpeggiatorPluginDemo.h

@@ -79,7 +79,7 @@ public:
 
     void processBlock (AudioBuffer<float>& buffer, MidiBuffer& midi) override
     {
-        // the audio buffer in a midi effect will have zero channels!
+        // A pure MIDI plugin shouldn't be provided any audio data
         jassert (buffer.getNumChannels() == 0);
 
         // however we use the buffer to get timing information

modules/juce_audio_processors/processors/juce_AudioProcessorGraph.cpp

@@ -268,7 +268,16 @@ private:
             for (int i = 0; i < totalChans; ++i)
                 audioChannels[i] = c.audioBuffers[audioChannelsToUse.getUnchecked (i)];
 
-            AudioBuffer<FloatType> buffer (audioChannels, totalChans, c.numSamples);
+            auto numAudioChannels = [this]
+            {
+                if (const auto* proc = node->getProcessor())
+                    if (proc->getTotalNumInputChannels() == 0 && proc->getTotalNumOutputChannels() == 0)
+                        return 0;
+
+                return totalChans;
+            }();
+
+            AudioBuffer<FloatType> buffer (audioChannels, numAudioChannels, c.numSamples);
 
             const ScopedLock lock (processor.getCallbackLock());
 

modules/juce_audio_processors/processors/juce_AudioProcessorGraph.h

@@ -155,17 +155,27 @@ public:
         void prepare (double newSampleRate, int newBlockSize, AudioProcessorGraph*, ProcessingPrecision);
         void unprepare();
 
+        bool hasNoConnections() const noexcept   { return inputs.isEmpty() && outputs.isEmpty(); }
+
         template <typename Sample>
         void processBlock (AudioBuffer<Sample>& audio, MidiBuffer& midi)
         {
+            if (hasNoConnections())
+                return;
+
             const ScopedLock lock (processorLock);
+
             processor->processBlock (audio, midi);
         }
 
         template <typename Sample>
         void processBlockBypassed (AudioBuffer<Sample>& audio, MidiBuffer& midi)
         {
+            if (hasNoConnections())
+                return;
+
             const ScopedLock lock (processorLock);
+
             processor->processBlockBypassed (audio, midi);
         }