Fix crash in adoptNode with attribute references

I forgot to also update the document reference of attributes, so when
there is no document reference anymore from a variable, but still a

Fix crash in adoptNode with attribute references

I forgot to also update the document reference of attributes, so when
there is no document reference anymore from a variable, but still an
attribute, this can crash. Fix it by also updating the document
references for attributes.

Closes GH-13002.

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