fix: handle null and undefined prop values

fixes #1

index-test.js

@@ -152,11 +152,27 @@ describe(`reactElementToJSXString(ReactElement)`, () => {
     }).toThrow('react-element-to-jsx-string: Expected a ReactElement');
   });
 
-  it('ignores object keys order (sortobject)', () => {
+  it(`ignores object keys order (sortobject)`, () => {
     expect(
       reactElementToJSXString(<div o={{a: 1, b: 2}}/>)
     ).toEqual(
       reactElementToJSXString(<div o={{b: 2, a: 1}}/>)
     );
   });
+
+  it(`reactElementToJSXString(<div a={null} />`, () => {
+    expect(
+      reactElementToJSXString(<div a={null} />)
+    ).toEqual(
+      reactElementToJSXString(<div a={null} />)
+    );
+  });
+
+  it(`reactElementToJSXString(<div a={undefined} />`, () => {
+    expect(
+      reactElementToJSXString(<div a={undefined} />)
+    ).toEqual(
+      reactElementToJSXString(<div a={undefined} />)
+    );
+  });
 });

index.js

@@ -117,7 +117,7 @@ function recurse({lvl, inline}) {
 
 function stringifyObject(obj) {
   // sortobject fails on some types, like regex
-  if (Object.keys(obj).length > 0) {
+  if (obj && Object.keys(obj).length > 0) {
     obj = sortobject(obj);
   }