Merge pull request #8167 from marmelab/fix-parse-doc

fix parse  doc

Author: fzaninotto

docs/Inputs.md

@@ -339,7 +339,7 @@ import { TextInput } from 'react-admin';
 const TextInputWithNullEmptyValue = props => (
     <TextInput
         {...props}
-        parse={ v => typeof v === 'string' && v.length === 0 ? null : v }
+        parse={ v =>  v === '' ? null : v }
     />
 );