Fix output_parser for generate_answer_node

[bezineb5]

This is a fix for #788 It looks like this case was forgotten during a previous refactoring.

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:tada: This PR is included in version 1.31.0 :tada:

The release is available on:

• v1.31.0
• GitHub release

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[github-actions[bot]]

:tada: This PR is included in version 1.31.0-beta.1 :tada:

The release is available on:

• v1.31.0-beta.1
• GitHub release

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