Set procedure param missing

src/blocks/scratch3_procedures.js

@@ -54,15 +54,19 @@ class Scratch3ProcedureBlocks {
     argumentReporterStringNumber (args, util) {
         const value = util.getParam(args.VALUE);
         if (value === null) {
-            return '';
+            // When the parameter is not found in the most recent procedure
+            // call, the default is always 0.
+            return 0;
         }
         return value;
     }
 
     argumentReporterBoolean (args, util) {
         const value = util.getParam(args.VALUE);
         if (value === null) {
-            return false;
+            // When the parameter is not found in the most recent procedure
+            // call, the default is always 0.
+            return 0;
         }
         return value;
     }

src/engine/thread.js

@@ -349,6 +349,7 @@ class Thread {
             if (frame.params.hasOwnProperty(paramName)) {
                 return frame.params[paramName];
             }
+            return null;
         }
         return null;
     }