Fix bitcoin-qt RPC when -server is used

Summary:
The bug was introduced in D7990. The node name usually designates a
node context, but not in this case.

Test Plan:

ninja all check-all

Without this diff, returns a Node context not found error:

./src/qt/bitcoin-qt -server
./src/bitcoin-cli getnetworkinfo

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D8652