Initialize position parameter in MasterImageCommandArgument to None

[brucetony]

A user locked an analysis without a entrypoint file resulting in this error form hub-adapter when trying to get the analysis nodes via the hub client:

pydantic_core._pydantic_core.ValidationError: 1 validation error for ResourceList[AnalysisNode]
data.3.analysis.image_command_arguments.0.position
  Field required [type=missing, input_value={'value': '-u'}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.12/v/missing

Both { value: "xxx" } or { value: "xxx", postion: "after" | "before" } is valid on the hub side, but the MasterImageCommandArgument requires the position argument and should be changed to the following:

class MasterImageCommandArgument(te.TypedDict):
    value: str
    position: t.Literal["before", "after"] | None = None