fix: check for undefined/null children (#136)

### Summary

Check for undefined/null values, in `getByText` nodes.

Fixes #135 

### Test plan

Passing tests on repro repository https://github.com/wKovacs64/rntl-161-repro.

Have no idea how to create a unit test, to test this behavior.

cc @thymikee @wKovacs64

src/helpers/getByAPI.js

@@ -20,8 +20,10 @@ const getNodeByText = (node, text) => {
     const isTextComponent =
       filterNodeByType(node, Text) || filterNodeByType(node, TextInput);
     if (isTextComponent) {
-      const textChildren = React.Children.map(node.props.children, child =>
-        child.toString()
+      const textChildren = React.Children.map(
+        node.props.children,
+        // In some cases child might be undefined
+        child => (child ? child.toString() : '')
       );
       if (textChildren) {
         const textToTest = textChildren.join('');