Fix TrendHistoryView trying to create shape layers with NaNs

Tusker/Views/TrendHistoryView.swift

@@ -48,6 +48,9 @@ class TrendHistoryView: UIView {
         }
         
         let maxUses = history.max(by: { $0.uses < $1.uses })!.uses
+        guard maxUses > 0 else {
+            return
+        }
         
         // remove old layers if this view is being re-used
         layer.sublayers?.forEach { $0.removeFromSuperlayer() }