controller string should be populated to be more consistent with other DID implementations

[troyronda]

Re: #1009 (comment)

#1006 (comment)
@OR13 wrote:

I strongly recommend against returning the empty string for the controller property.... if makes sidetree look different than every other did method, and its not guaranteed to work accross JSON-LD implementations... the controller property should match the didDocument.id.

I agree - it is better to populate the controller property rather than leave it empty.

@thehenrytsai @OR13 @csuwildcat @sandrask @isaacJChen

[OR13]

Is this a v1 issue?

[OR13]

For example: https://github.com/transmute-industries/sidetree.js/blob/main/packages/test-vectors/src/sidetree-universal-wallet/wallet-operation.json#L37

^ we support controller like this.

[llorllale]

This isn't a consistency issue... this is a compliance issue.

From did-core:

controller
The value of the controller property MUST be a string that conforms to the rules in § 3.1 DID Syntax.