Fix problem with check target numbers <= 0

The fix has 2 components:
1. The logic for evaluating checks now supports check target numbers<= 0
   by still using a single die in this case
2.  The CheckFactory sets the check target number to 0 even if it would
   be < 0. This is necessary because negative numbers would interfer
   with foundry's math evaluation in rolls and would not be picked up
   correctly.

src/module/rolls/check-evaluation.ts

@@ -113,5 +113,5 @@ function evaluateDiceWithSubChecks(
 }
 
 export function getRequiredNumberOfDice(checkTargetNumber: number): number {
-    return Math.ceil(checkTargetNumber / 20);
+    return Math.max(Math.ceil(checkTargetNumber / 20), 1);
 }

src/module/rolls/check-factory.ts

@@ -45,8 +45,8 @@ class CheckFactory {
         );
     }
 
-    createCheckTargetNumberModifier(): string | null {
-        return "v" + (this.checkTargetNumber + this.gmModifier);
+    createCheckTargetNumberModifier(): string {
+        return "v" + Math.max(this.checkTargetNumber + this.gmModifier, 0);
     }
 
     createCoupFumbleModifier(): string | null {