fix(parser): reactnode should make proptype optional

Author: merceyz

src/parser.ts

@@ -211,7 +211,7 @@ export function parseFromProgram(filePath: string, program: ts.Program) {
           return t.elementNode('element');
         }
         case 'React.ReactNode': {
-          return t.elementNode('node');
+          return t.unionNode([t.elementNode('node'), t.undefinedNode()]);
         }
       }
     }

src/types/props/union.ts

@@ -7,9 +7,23 @@ export interface UnionNode extends Node {
 }
 
 export function unionNode(types: Node[]): UnionNode {
+  const flatTypes: Node[] = [];
+
+  flattenTypes(types);
+
+  function flattenTypes(nodes: Node[]) {
+    nodes.forEach(x => {
+      if (isUnionNode(x)) {
+        flattenTypes(x.types);
+      } else {
+        flatTypes.push(x);
+      }
+    });
+  }
+
   return {
     type: typeString,
-    types: types,
+    types: flatTypes,
   };
 }