CRM-2604 Duplicate check fails and throws errors if there is no content in a duplicate check field.

Reproduced in 1.9 and the current 2.0 alpha sandbox.

If the Duplicate Contact Rules include a field that is not used by any record, then Find Duplicate Contacts will fail and throw nasty looking errors on screen, such as:
"Sorry. A non-recoverable error has occurred.

Database Error Code: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'AS 'match' FROM civicrm_address param WHERE param.contact_id = 1' at line 1, 1064

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Error Details:

Array
(
[callback] => Array
(
[0] => CRM_Core_Error
[1] => handle
)

[code] => -2
[message] => DB Error: syntax error
[mode] => 16
[debug_info] => SELECT param. AS 'match' FROM civicrm_address param
WHERE param.contact_id = 1 [nativecode=1064 ** You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'AS 'match' FROM civicrm_address param
WHERE param.contact_id = 1' at line 1]
[type] => DB_Error
[user_info] => SELECT param. AS 'match' FROM civicrm_address param
WHERE param.contact_id = 1 [nativecode=1064 ** You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'AS 'match' FROM civicrm_address param
WHERE param.contact_id = 1' at line 1]
[to_string] => [db_error: message="DB Error: syntax error" code=-2 mode=callback callback=CRM_Core_Error::handle prefix="" info="SELECT param. AS 'match' FROM civicrm_address param
WHERE param.contact_id = 1 [nativecode=1064 ** You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'AS 'match' FROM civicrm_address param
WHERE param.contact_id = 1' at line 1]"]
)"

To reproduce:
1> Change the Duplicate Contact Rules to include a field that is empty for all records of the selected type. In the sandbox, it took a little looking, but apparently Supplemental Address 2 is unused for all Individual records.
2> Do a Find Duplicate Contacts using the relevant rule-set.

Ideally, CiviCRM would simply disregard fields that are totally empty during the check. Failing this, a meaningful, human-readable error suggesting that the rules include empty fields would save some confused CiviCRM administrators.