encoding/json: fix using omitempty in combination with MarshalJSON

[inteon]

The omitempty option does not support objects with a MarshalJSON function.
This means that a field annotated with the omitempty tag can still contain a null, 0, false, "" or [] value and will never be omitted.
This PR fixes this issue: if the MarshalJSON function returns an empty value, that value is omitted.

See #45669 (comment) for more background information.

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Message from Daniel Martí:

Patch Set 1: Hold+1

(1 comment)

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Message from Gopher Robot:

Patch Set 1:

Congratulations on opening your first change. Thank you for your contribution!

Next steps:
A maintainer will review your change and provide feedback. See
https://go.dev/doc/contribute#review for more info and tips to get your
patch through code review.

Most changes in the Go project go through a few rounds of revision. This can be
surprising to people new to the project. The careful, iterative review process
is our way of helping mentor contributors and ensuring that their contributions
have a lasting impact.

During May-July and Nov-Jan the Go project is in a code freeze, during which
little code gets reviewed or merged. If a reviewer responds with a comment like
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for more details.

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[gopherbot]

Message from Daniel Martí:

Patch Set 1: Hold+1

(1 comment)

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