fix: allow unencrypted PEM

[JasonPowr]

Allow ReadPrivateKeyFile and FromProto (via PEMKeyFile) to accept unencrypted PEM private key files by removing the empty password guard. UnmarshalPrivateKey already handled both cases correctly.

Checklist

• I have updated the CHANGELOG.
• I have updated documentation accordingly (including the feature implementation matrix).

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[AlCutter]

Hi @JasonPowr, thanks for the PR.

Could you drop the merge commit and just rebase the first commit onto an up-to-date master?

[JasonPowr]

@AlCutter Sure done :)

[AlCutter]

/gcbrun