Fix declared flags option

[ocavue]

When setting an option with the type of ParameterType.Flags, if this._values[declaration.name] doesn't exist yet, TypeDoc will throw [error] Cannot convert undefined or null to object. This PR fixes it by assigning an empty object when it is undefined.

This PR also improves the logging by adding some extra information about the current key when setValue method throws an error.

[Gerrit0]

It shouldn't be possible for _values[name] to be undefined for flags at this point... do you have a repro example you can share?

[ocavue]

https://stackblitz.com/github/issueset/typedoc-markdown-flags-issue?file=config%2Ftypedoc-base.json

You can open this link to see the [error] Cannot convert undefined or null to object error.

I'm using the namedAnchors option from typedoc-plugin-markdown. Maybe this is an issue on the typedoc-plugin-markdown side.

[Gerrit0]

Ah, looks like I messed up in Options.copyForPackage, that function copies the declarations from the root options object, but neglects setting their default values. typedoc-plugin-markdown is doing the right thing.

[Gerrit0]

I don't think I want to add the logging to all of the readers, the errors thrown from the Options class (especially those that can be caused by bad configuration) should already have the option name in them, will review those to make sure they look right.

[ocavue]

thanks for your fix！

[Gerrit0]

Thank you! Your repro made it quickly obvious what the problem was once I looked at it :)