Bug 798448 - Column 'num' cannot be null

Convert libofx's data.check_number and data.reference_number to "" if
they are any case variation of "null".

Note that SQL backends will not permit the word NULL as a value for the
number field if it's entered from the GUI, but this can be worked around
by quoting it as in Scheme: 'Null *will* work.

gnucash/import-export/ofx/gnc-ofx-import.c

@@ -614,7 +614,14 @@ int ofx_proc_transaction_cb(struct OfxTransactionData data, void *user_data)
             /* set tran-num and/or split-action per book option */
             if (data.check_number_valid)
             {
-                gnc_set_num_action(transaction, split, data.check_number, NULL);
+                /* SQL will correctly interpret the string "null", but
+                 * the transaction num field is declared to be
+                 * non-null so substitute the empty string.
+                 */
+                const char *num_value =
+                    strcasecmp (data.check_number, "null") == 0 ? "" :
+                    data.check_number;
+                gnc_set_num_action(transaction, split, num_value, NULL);
             }
             else if (data.reference_number_valid)
             {