fix(manager/dockerfile): handle single-quoted empty ARG default value

[youdie006]

Closes #44497.

Renovate's Dockerfile ARG value unquoting only stripped double quotes, so ARG FOO='' kept the literal '' and FROM ${FOO}node:... resolved to ''node (which fails lookup). This extends the unquoting in lib/modules/manager/dockerfile/extract.ts to strip single quotes the same way, so both '' and "" empty values yield an empty string.

Test plan

Added handles FROM with single-quoted empty ARG default value in extract.spec.ts using the issue's exact repro (ARG DOCKER_HUB_MIRROR=''). Red before the fix (depName: "''node"), green after. Full dockerfile manager suite: 82 passed; the changed lines are at 100% coverage; prettier / oxlint / biome clean.

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AI disclosure: this PR was drafted with Claude Code (AI-assisted). Tests were run red->green (82 passed) with coverage/lint verified.

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[github-actions]

Hi there,
We notice that the Pull Request you've created doesn't quite follow our template.
Please update the PR description to follow our template.
This improves the maintainers' time to triage, as key information is clearly indicated in the format we expect.
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