Document naming rule with no trailing digits

**Is your feature request related to a problem? Please describe.**
When we wanted to add series for a particular document where the digits are in between (not at the end), document naming rule is mandating to put digits.

For example, I need to put a series like .YY.-TM-.####.-.MM.DD

generated names should be like:

23-TM-0001-0508
23-TM-0002-0508
23-TM-0003-0508

But as digits are mandatory in document naming rule, even if we put 0 the series is generating as below:

23-TM-0001-05081
23-TM-0002-05082
23-TM-0003-05083

**Describe the solution you'd like**
If digits is 0, do not append any numbers to the prefix

**Describe alternatives you've considered**
Document naming setting can be used. But sometimes custom series are getting wiped off on update. Also there we cannot put any rules. So again will have to write a script to do the same.

**Additional context**
ERPNext: v14.33.2 (HEAD)
Frappe Framework: v14.43.0 (HEAD)