Importing custom fields with Null breaks during call to deprecated api

Issue with treatment of NULL values during import of contacts related to custom fields.

If there is a single custom field present as NULL in the CSV file, specifying the mapping doesn't make a difference to the import. The contact is imported successfully.

Issue arises when there are multiple custom fields and any one of them has a NULL value. The entire record is skipped and nothing is updated without generating an error message.

After debugging, found that the error is being generated due to a typecheck on the value of the custom field being imported. The process is being aborted at the following LOC: 1670 of CRM/Import/Parser/Contact.php

A check on the value of the custom field is being done by a function in the file 'api/v3/DeprecatedUtils.php'. The question thus arises, is the check being done right owing to the file being a part of a deprecated set of functions?

Currently applied a workaround by unsetting NULL values before parsing the custom fields. Please find the patch attached.