title: TextFormField validation shows no any effect for the validator used
body: <p>Hi I am currently trying to validate a text and number field and using TextFormField widget but even though i have defined validator, it gives no any effect.</p>
  <pre><code>   TextFormField(
                controller: _zipcode,
                keyboardType: TextInputType.number,
                validator: (value) {
                  if (value==null || value.isEmpty ) {
                    return &quot;Enter correct value&quot;;
                  } else {
                    return null;
                  }
                },
                decoration: InputDecoration(
                    labelText: 'Mileage as shown in Odometer'),
              )
  </code></pre>