iOS AUv3: MIDIOutputEventBlock can be null

Hey JUCE team :slight_smile: Just a small report: I noticed that my plug-in AUv3 is crashing in MultitrackStudio on iOS. The reason is that the MIDIOutputEventBlock property is optional and can be null, but the JUCE AUv3 wrapper calls it without checking. Inserting an if (midiOut) seems to solve it.