Fix issue and provide convenience method

app/src/main/java/net/pokeranalytics/android/calculus/Calculator.kt

@@ -94,6 +94,10 @@ class Calculator {
 
             var results: ComputedResults = ComputedResults()
 
+            if (sessions.size == 0) {
+                return results
+            }
+
             var sum: Double = 0.0
             var totalHands: Double = 0.0
             var bbSum: Double = 0.0

app/src/main/java/net/pokeranalytics/android/calculus/Computable.kt

@@ -97,6 +97,13 @@ class ComputedResults() {
         }
     }
 
+    /**
+     * Returns the number of computed stats
+     */
+    fun numberOfStats() : Int {
+        return this._computedStats.size
+    }
+
 }
 
 class Point(x: Double, y: Double) {