parameter.cache() will change an invalid cache to valid, even for parameters with `get_cmd=None`

Steps to reproduce

p = Parameter("name", set_cmd=None)
assert not p.cache.valid
p.cache()
assert not p.cache.valid

p = Parameter("name", set_cmd=None)
p.set(5)
assert p.cache.valid
p.cache.invalidate()
assert not p.cache.valid
p.cache()
assert not p.cache.valid

the last assert will fail in both cases

Expected behaviour

As we did not implement a get_cmd, parameter.cache() always returns the last cached value,even though it is not valid (this is fine).
Querying the cache should leave it marked as invalid, unless we get information about the parameter from another place, i.e. unless a get_cmd is implemented.

Actual behaviour

parameter.cache() will always update the cache, switching an invalid cache to valid. See the red path in the flow diagram:

Comment

It is possible that using both cache.invalidate() and get_cmd=None together is not intended and considered user error. In this case, this should be documented.

System

windows
qcodes 0.33.0

Start at the Parameter cache() and cache.invalidate() entry points and reproduce the two examples from the issue on Windows or the available test environment. Add coverage for parameters with get_cmd=None, then verify that querying an invalid cache leaves it invalid while still returning the last cached value.