Fix test: property_test_rpc_server

Test: property_test_rpc_server File: tezt/long_tests/qcheck_rpc.ml Alert: File "src/lib_openapi/convert.ml", line 188, characters 8-14: Assertion failed

So far I have identified 4 issues with qcheck_rpc.ml. Instead of trying to solve them all in a single PR, I'll do a PR for each issue:

• Bad handling of null object in json schema parsing -> !5997 (merged)
• Spurious warning messages of the form expected path /foo/bar but got /foo/bar -> !5998 (merged)
• Stack overflows / segmentation faults when parsing json schemas -> !5999 (merged)
• Tests of server node alive-ness returns false negatives