[bug] sorting custom fields by default value does not work

[P6g9YHK6]

Server Info (please complete the following information):

• OS: Debian 12
• Browser: any
• RMM Version (as shown in top left of web UI): 0.18.2

Installation Method:

• Standard
• Standard with --insecure flag at install
• Docker

Agent Info (please complete the following information):

• Agent version (as shown in the 'Summary' tab of the agent from web UI): NA
• Agent OS: NA

Describe the bug
sorting by default value does not work

To Reproduce
Steps to reproduce the behavior:

1. Go to custom fields
2. Click on default value
3. See error

Expected behavior
proper sorting

Screenshots

[P6g9YHK6]

another one:

[wh1te909]

sort won't work if there are null/empty string fields. we'll need to implement a custom sort function for this