fix(tinytype): toJSON correctly serialises null and undefined values

spec/TinyType.spec.ts

@@ -327,6 +327,26 @@ describe('TinyType', () => {
                     });
                 });
 
+                it('should serialise null and undefined', () => {
+                    interface NotesType {
+                        nullValue: any;
+                        undefinedValue: any;
+                    }
+
+                    class Notes extends TinyTypeOf<NotesType>() {
+                    }
+
+                    const notes = new Notes({
+                        nullValue: null,
+                        undefinedValue: undefined,
+                    });
+
+                    expect(notes.toJSON()).to.deep.equal({
+                        nullValue: null,
+                        undefinedValue: undefined,
+                    });
+                });
+
                 it(`should JSON.stringify any object that can't be represented in a more sensible way`, () => {
                     class TT extends TinyTypeOf<number>() {
                     }

src/TinyType.ts

@@ -178,6 +178,13 @@ function isSerialisableNumber(value: unknown): value is number {
 }
 
 function isSerialisablePrimitive(value: unknown): value is string | boolean | number | null | undefined {
-    return ['string', 'boolean', 'null', 'undefined'].includes(typeof value)
-        || isSerialisableNumber(value);
+    if (['string', 'boolean'].includes(typeof value)) {
+        return true;
+    }
+
+    if (value === null || value === undefined) {
+        return true;
+    }
+
+    return isSerialisableNumber(value);
 }