feat: fix empty string values converted to NULL in PostgreSQL

[Copilot]

Empty strings in Casbin rules were being converted to NULL in PostgreSQL, making it impossible to distinguish between intentionally empty values and wildcards in filters.

Changes

Database mapping

• Added pg:",use_zero" tags to V0-V5 fields in CasbinRule struct to prevent go-pg from converting empty strings to NULL

Rule serialization

• Updated String(), toStringPolicy(), and queryString() methods to preserve empty strings up to the last non-empty value
• Added getValues() and getLastNonEmptyIndex() helpers to avoid duplicate slice allocations

Behavior

Empty strings in the middle of rules are now preserved:

// Before: stored as NULL, couldn't be matched
e.AddPolicy("alice", "", "read")  // Failed to store/retrieve correctly

// After: stored as empty string, exact match works
e.AddPolicy("alice", "", "read")  
e.RemovePolicy("alice", "", "read")  // ✓ Matches and removes

// Wildcard filtering still works
e.RemoveFilteredPolicy(0, "", "data1")  // ✓ Matches all v0 values

Trailing empty strings are trimmed by Casbin per its design (not changed).

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• esm.ubuntu.com
  • Triggering command: /usr/lib/apt/methods/https /usr/lib/apt/methods/https (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Is casbin seriously not supporting empty string values?</issue_title>
<issue_description>Is casbin seriously not supporting empty string values?
I get [null] in postgres every single time I try to use an empty string
using casbin's official postgres adapter github.com/casbin/casbin-pg-adapter
Seems like a massive issue
What if I want to differentiate between a wildcard and a purposefully omitted value?
It just turns into a [null] and breaks casbin</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Is casbin seriously not supporting empty string values? #51

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