fix: guard default value comparison against types with strict __eq__

[Bahtya]

Problem

Fixes #3298

When a non-string default value (e.g. semver.Version) is used with an @click.option, Click tries to generate help text and compares the default value against an empty string using ==. Types like semver.Version implement __eq__ to only accept strings that parse as valid semver, so Version(1,0,0) == "" raises ValueError.

Root Cause

In core.py line ~3113:

elif default_value == "":
    default_string = "\"\""

This equality check is triggered for any type of default value, not just strings. For types with strict __eq__ implementations, this crashes.

Fix

Add an isinstance(default_value, str) guard before the == "" check, so non-string types fall through to the str(default_value) branch which works correctly for all types.

Testing

import click
from semver import Version

class SemverType(click.ParamType):
    name = "semver"
    def convert(self, value, param, ctx):
        if isinstance(value, Version):
            return value
        return Version.parse(value)

@click.command()
@click.option("--version", type=SemverType(), default=Version(1, 0, 0))
def cli(version):
    click.echo(f"Version: {version}")

# This now works without ValueError
from click.testing import CliRunner
runner = CliRunner()
result = runner.invoke(cli, ["--help"])

Existing test suite passes (331 passed; 1 pre-existing failure unrelated to this change).

[kdeldycke]

AI slop duplicating: #3299