Network: Prevent image loading for hidden cluster nodes (#3492)

Fixes #3483.

If `shape: 'image'` is defined as option, this was used for initializing hidden cluster nodes during layout initialization.
However, because no image is being passed, this led to an exception.

This fix prevens shape `image` from being used for hidden cluster nodes. In addition, some extra fields for these nodes are
now overridden for some performance improvament.

lib/network/modules/LayoutEngine.js

@@ -561,13 +561,32 @@ class LayoutEngine {
       if (positionDefined < 0.5 * indices.length) {
         let MAX_LEVELS = 10;
         let level = 0;
-        let clusterThreshold = 150;
-        // Performance enhancement, during clustering edges need only be simple straight lines.
+        let clusterThreshold = 150;  // TODO add this to options
+
+        //
+        // Define the options for the hidden cluster nodes
         // These options don't propagate outside the clustering phase.
+        //
+        // Some options are explicitly disabled, because they may be set in group or default node options.
+        // The clusters are never displayed, so most explicit settings here serve as performance optimizations.
+        //
+        // The explicit setting of 'shape' is to avoid `shape: 'image'`; images are not passed to the hidden
+        // cluster nodes, leading to an exception on creation.
+        //
+        // All settings here are performance related, except when noted otherwise.
+        //
         let clusterOptions = {
+          clusterNodeProperties:{
+            shape: 'ellipse',       // Bugfix: avoid type 'image', no images supplied
+            label: '',              // avoid label handling
+            group: '',              // avoid group handling
+            font: {multi: false},   // avoid font propagation
+          },
           clusterEdgeProperties:{
+            label: '',              // avoid label handling
+            font: {multi: false},   // avoid font propagation
             smooth: {
-              enabled: false
+              enabled: false        // avoid drawing penalty for complex edges
             }
           }
         };

lib/network/modules/components/shared/Label.js

@@ -64,7 +64,7 @@ class Label {
 
     this.initFontOptions(options.font);
 
-    if (options.label !== undefined) {
+    if (options.label !== undefined && options.label !== null && options.label !== '') {
       this.labelDirty = true;
     }