fix(postgrest): validate empty or invalid relation names in Postgrest…

[sofmega]

🔍 Description

This PR validates the relation argument passed to PostgrestClient.from().
Previously, calling:

supabase.from('')

would generate an invalid URL (ending with //) and result in unclear or confusing PostgREST or network errors.
This validation provides clearer developer feedback and avoids silent failures.

What changed?

• Added a validation check in PostgrestClient.from() to ensure the relation name:
  • is a string
  • is not empty
  • is not only whitespace

If validation fails, the client now throws:

Invalid relation name: relation must be a non-empty string.

Why was this change needed?

Developers frequently make small mistakes such as using an empty string for the relation.
Without validation, this results in unclear and difficult-to-debug errors from downstream fetch calls.

This PR improves DX (developer experience) by catching the issue early and providing a helpful error message.

Closes # (not applicable)

---

📸 Screenshots/Examples

Before

supabase.from('')
// ❌ Leads to confusing network errors or malformed URLs

After

supabase.from('')
// ❌ Throws: "Invalid relation name: relation must be a non-empty string."

---

🔄 Breaking changes

• This PR contains no breaking changes

---

📋 Checklist

• I have read the Contributing Guidelines
• My PR title follows the conventional commit format: <type>(<scope>): <description>
• I have run npx nx format to ensure consistent code formatting
• I have added tests for new functionality (if applicable)
• I have updated documentation (not applicable)

---

📝 Additional notes

This enhancement helps prevent silent URL errors, improves developer feedback, and aligns with the existing validation behavior in other Supabase JavaScript client modules.