ensure maxLabelWidth is a number

Co-authored-by: pranavm2109 <mishrap@dickinson.edu>

packages/mermaid/src/diagrams/user-journey/journeyRenderer.ts

@@ -22,6 +22,10 @@ function drawActorLegend(diagram) {
   maxWidth = 0;
   let yPos = 60;
 
+  if (isNaN(maxLabelWidth)) {
+    throw new Error('maxLabelWidth must be a number');
+  }
+
   Object.keys(actors).forEach((person) => {
     const colour = actors[person].color;
     const circleData = {