Avoid NullPointerException in isEnabled method

Added null validation of readonly variable in protected boolean  isEnabled(String locator) method. Please see issue #53: https://github.com/MarkusBernhardt/robotframework-selenium2library-java/issues/53

Basically, this avoids a NullPointerException if the html element doesn't have the "readonly" attribute.

Use this Robot test to check it:
*** Settings ***
Library  Selenium2Library

*** Test Cases ***
```
Test Should Be Pass and Should Not Throw a RuntimeException
    Open Browser    http://www.zkoss.org/zkdemo/input/form_sample    firefox    mainbrowser
    Wait Until Keyword Succeeds  10 sec  1 sec  Element Should Be Enabled  //input[../../../td[1]//span[contains(text(), 'Password')]]
    Close All Browsers
```

I am available for any questions you have.

Thank you