tests: qemumonitorjson: Old and empty test case for node name detection
    
    Prior to qemu 2.5 the node names would not be generated, thus would be
    missing from 'query-blockstats' and 'query-named-block-nodes'. Test that
    the code correctly detects nothing.
    
    Additionally make sure that a VM without disks does not cause problems.
    
    The test case change is necessary as our test file checker does not play
    well with empty files.