NextEventQueue: do not crash if no events are defined

This is most likely an operator error, but we should handle it gracefully.

12:18:56.255 E n.n.BaseCalc - neurord.numeric.grid.AdaptiveGridCalc@6c9f5c0d: failed (seed=123)
12:18:56.255 E n.SDCalc - Trial 0 failed!
 java.lang.ArrayIndexOutOfBoundsException: 0
        at neurord.numeric.grid.NextEventQueue$PriorityTree.first(NextEventQueue.java:142) ~[classes/:?]
        at neurord.numeric.grid.NextEventQueue.advance(NextEventQueue.java:1999) ~[classes/:?]
        at neurord.numeric.grid.AdaptiveGridCalc.advance(AdaptiveGridCalc.java:108) ~[classes/:?]
        at neurord.numeric.grid.GridCalc._run(GridCalc.java:151) ~[classes/:?]
        at neurord.numeric.BaseCalc.run(BaseCalc.java:121) ~[classes/:?]
        at neurord.SDCalc.run(SDCalc.java:87) [classes/:?]
        at neurord.StochDiff.main(StochDiff.java:184) [classes/:?]

↓

12:26:26.944 W n.n.g.NextEventQueue - Event queue is empty — no diffusion, reaction, or stimulation events

src/main/java/neurord/numeric/grid/NextEventQueue.java

@@ -139,7 +139,11 @@ public int compare(T a, T b) {
         }
 
         T first() {
-            return this.nodes[0];
+            if (this.nodes.length == 0)
+                return null;
+            T node = this.nodes[0];
+            assert node != null;
+            return node;
         }
 
         void reposition(String prefix, T node) {
@@ -1993,12 +1997,20 @@ public static NextEventQueue create(int[][] particles,
      *
      * @returns Time of soonest event.
      */
+    private static boolean _warned_empty = false;
     public double advance(double time, double tstop, double timelimit,
                           int[][] eventStatistics,
                           List<IGridCalc.Happening> events) {
-        NextEvent ev = this.queue.first();
-        assert ev != null;
-        double now = ev.time;
+        final NextEvent ev = this.queue.first();
+        final double now;
+        if (ev == null) {
+            if (!_warned_empty) {
+                log.warn("Event queue is empty — no diffusion, reaction, or stimulation events");
+                _warned_empty = true;
+            }
+            now = Double.POSITIVE_INFINITY;
+        } else
+            now = ev.time;
         assert now >= time: ev;
 
         if (now > tstop) {