EditDiskModal: Properly initialize Cache selector

`cacheMode` is null if the domain XML does not specify a value, but the
`value` should always point to an actual valid option. Fall back to
"default".

src/components/vm/disks/diskEdit.jsx

@@ -67,7 +67,7 @@ const CacheRow = ({ onValueChanged, dialogValues, idPrefix, shutoff }) => {
             <FormSelect id={`${idPrefix}-cache-mode`}
                         onChange={(_event, value) => onValueChanged('cacheMode', value)}
                         isDisabled={!shutoff}
-                        value={dialogValues.cacheMode}>
+                        value={dialogValues.cacheMode ?? "default"}>
                 {diskCacheModes.map(cacheMode => {
                     return (
                         <FormSelectOption value={cacheMode} key={cacheMode}

test/machineslib.py

@@ -433,7 +433,6 @@ def setUp(self):
             "Tried changing state of a disconnected RFB object",
             "Failed to get libvirt version from the dbus API:.*Cannot recv data: Connection reset by peer",
             # FIXME: React errors on /devel scenario; these are actual bugs!
-            "Warning: .* prop on .* should not be null.",
             "Warning: Received .* for a non-boolean attribute",
             'Warning: Each child in a list should have a unique "key" prop',
             "Warning: React does not recognize the .* prop on a DOM element",