Description of problem:
Obsolete custom fields with type of 'Integer' or 'Number' can not be deleted.

Version-Release number of selected component (if applicable):
4.4.9039-3

How reproducible:
100%

Steps to Reproduce:
1. Create two fields through Administration -> Custom Fields -> Add a new custom field. Details is as below:
Name: cf_test1, Type: Integer
Name: cf_test2, Type: Number
Both have 'Can be set on bug creation' and 'Displayed in bugmail for new bugs' checked and 'product is TCMS' set for 'Field only appears when'.
2. Modify two fields with 'Is obsolete' checked.
3. Go back to custom field list page, click 'Delete' and confirm the deletion.

Actual results:
DBD::Pg::db selectrow_array failed: ERROR:  invalid input syntax for integer: ""
LINE 1: ...T(*) FROM bugs WHERE cf_test1 IS NOT NULL AND cf_test1 != ''
                                                                     ^ [for Statement "SELECT COUNT(*) FROM bugs WHERE cf_test1 IS NOT NULL AND cf_test1 != ''"] at /var/www/html/bugzilla/Bugzilla/Field.pm line 983
	Bugzilla::Field::remove_from_db('Bugzilla::Field=HASH(0x7f12ef4a4118)') called at /var/www/html/bugzilla/editfields.cgi line 156
	ModPerl::ROOT::Bugzilla::ModPerl::ResponseHandler::var_www_html_bugzilla_editfields_2ecgi::handler('Apache2::RequestRec=SCALAR(0x7f12f03f2570)') called at /usr/lib64/perl5/vendor_perl/ModPerl/RegistryCooker.pm line 204
	eval {...} called at /usr/lib64/perl5/vendor_perl/ModPerl/RegistryCooker.pm line 204
	ModPerl::RegistryCooker::run('Bugzilla::ModPerl::ResponseHandler=HASH(0x7f12efb2dda0)') called at /usr/lib64/perl5/vendor_perl/ModPerl/RegistryCooker.pm line 170
	ModPerl::RegistryCooker::default_handler('Bugzilla::ModPerl::ResponseHandler=HASH(0x7f12efb2dda0)') called at /usr/lib64/perl5/vendor_perl/ModPerl/Registry.pm line 31
	ModPerl::Registry::handler('Bugzilla::ModPerl::ResponseHandler', 'Apache2::RequestRec=SCALAR(0x7f12f03f2570)') called at /var/www/html/bugzilla/mod_perl.pl line 134
	Bugzilla::ModPerl::ResponseHandler::handler('Bugzilla::ModPerl::ResponseHandler', 'Apache2::RequestRec=SCALAR(0x7f12f03f2570)') called at -e line 0
	eval {...} called at -e line 0

Expected results:
Fields should be deleted successfully.