[Qt6 regression August->September] QNAM::get fails first request

Noticed it in QtWebEngine after updating from  ~August 19th dependencies  to ~September 16th dependencies.

Basically the first QNAM::get fails with ContentNotFoundError, and trying again in a wrapping function that also creates a new QNAM will make it keep failing every time, but reusing the QNAM and sending a second request with the same QNAM works on the second try.

See https://codereview.qt-project.org/c/qt/qtwebengine/+/314588 and https://testresults.qt.io/coin/api/results/qt/qtwebengine/765ef21ba67186b7912649649c4851add838e59f/MacOSMacOS_10_14x86_64MacOSMacOS_10_14x86_64Clangqtci-macos-10.14-x86_64-187515UseLegacyInstructions/158a3531ca539995c0e1d2561e661d3d0ef8e268/test_1600687056/log.txt.gz and
https://testresults.qt.io/coin/api/results/qt/qtwebengine/765ef21ba67186b7912649649c4851add838e59f/LinuxRHEL_7_6x86_64LinuxRHEL_7_6x86_64GCCqtci-linux-RHEL-7.6-x86_64-d93c62UseLegacyInstructions/cc41a2251595d87115f08601f68405c3f321d252/test_1600687054/log.txt.gz