Avoid crash validating event type with blank length

The `length_step` validator runs unconditionally for every record and
calls `length % program.schedule_interval`. When the length field is
left blank in the admin form, `length` is nil and `length %` raises
NoMethodError, returning a 500 Internal Server Error instead of a
regular validation failure.

Skip the divisor check unless `length` is numeric so that the
`numericality` validator on `:length` can produce a proper error
message, restoring the usual validation feedback.

spec/models/event_type_spec.rb

@@ -44,6 +44,13 @@ describe EventType do
       it 'is not valid when length is not multiple of LENGTH_STEP' do
         expect(build(:event_type, program: conference.program, length: 37)).not_to be_valid
       end
+
+      it 'is not valid when length is blank and does not raise during validation' do
+        event_type = build(:event_type, program: conference.program, length: nil)
+        expect { event_type.valid? }.not_to raise_error
+        expect(event_type).not_to be_valid
+        expect(event_type.errors[:length]).to be_present
+      end
     end
   end
 end