Return a default falsey value for unknown params based on param type

src/blocks/scratch3_procedures.js

@@ -15,8 +15,8 @@ class Scratch3ProcedureBlocks {
         return {
             procedures_definition: this.definition,
             procedures_call: this.call,
-            argument_reporter_string_number: this.param,
-            argument_reporter_boolean: this.param
+            argument_reporter_string_number: this.argumentReporterStringNumber,
+            argument_reporter_boolean: this.argumentReporterBoolean
         };
     }
 
@@ -47,8 +47,19 @@ class Scratch3ProcedureBlocks {
         }
     }
 
-    param (args, util) {
+    argumentReporterStringNumber (args, util) {
         const value = util.getParam(args.VALUE);
+        if (value === null) {
+            return '';
+        }
+        return value;
+    }
+
+    argumentReporterBoolean (args, util) {
+        const value = util.getParam(args.VALUE);
+        if (value === null) {
+            return false;
+        }
         return value;
     }
 }

test/unit/engine_thread.js

@@ -110,6 +110,8 @@ test('PushGetParam', t => {
     th.pushParam('testParam', 'testValue');
     t.strictEquals(th.peekStackFrame().params.testParam, 'testValue');
     t.strictEquals(th.getParam('testParam'), 'testValue');
+    // Params outside of define stack always evaluate to null
+    t.strictEquals(th.getParam('nonExistentParam'), null);
 
     t.end();
 });