study_consent_withdrawal either null or 0000-00-00 will work

[gluneau]

Same as in https://github.com/aces/IBIS/blob/master/tools/NDAR_MRI/restrict_ibis1.php

[codecov-io]

Current coverage is 25.80%

Merging #1523 into 16.04-dev will increase coverage by +1.84% as of 4e2129e

@@            16.04-dev   #1523   diff @@
=========================================
  Files             110     110       
  Stmts           18443   18468    +25
  Branches            0       0       
  Methods          1025    1025       
=========================================
+ Hit              4419    4766   +347
  Partial             0       0       
+ Missed          14024   13702   -322

Review entire Coverage Diff as of 4e2129e

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[driusan]

It's probably better to figure out why/where invalid dates are getting inserted and fix the source, but this works for now..