renderer/metal: don't draw with zero instances

src/renderer/Metal.zig

@@ -1227,6 +1227,10 @@ fn drawCells(
     buf: CellBuffer,
     len: usize,
 ) !void {
+    // This triggers an assertion in the Metal API if we try to draw
+    // with an instance count of 0 so just bail.
+    if (len == 0) return;
+
     // Use our shader pipeline
     encoder.msgSend(
         void,