rpc: remove mutable default argument

Using mutable objects (e.g. a dict) as default arguments to a function
can lead to unexpected behavior, as the same object will be used for
each call to that function.  For example:

def foo(bar, baz=[]):
	baz.append(bar)
	return baz

>>> foo(1)
[1]
>>> foo(2)
[1, 2]

We don't want params to be remembered between function calls, so they're
now default initialized to None and set to an empty dict in the body of
the function.

Signed-off-by: Konrad Sztyber <konrad.sztyber@intel.com>
Change-Id: I63c71b5424286658759f09aed2d30c529c2d1081
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/19807


Reviewed-by: Artur Paszkiewicz <artur.paszkiewicz@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Jim Harris <jim.harris@gmail.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>