Updated the check of empty array in the constraints tests.

[gabrieldemarmiesse]

Summary

We have many warnings looking like this:

tests/keras/constraints_test.py:75
  /home/travis/build/keras-team/keras/tests/keras/constraints_test.py:75: DeprecationWarning: The truth value of an empty array is ambiguous. Returning False, but in future this will result in an error. Use `array.size > 0` to check that an array is not empty.
    assert not l2[l2 > m * 2 + 1e-5]

This PR fixes this problem.

Related Issues

PR Overview

• This PR requires new unit tests [y/n] (make sure tests are included)
• This PR requires to update the documentation [y/n] (make sure the docs are up-to-date)
• This PR is backwards compatible [y/n]
• This PR changes the current API [y/n] (all API changes need to be approved by fchollet)