SafeMaskMaker fails with fixed_offset option

[adonath]

Gammapy version
v0.20

Bug description
When using the fixed_offset option and the "aeff-max" option the SafeMaskMaker fails with:

ValueError: observation argument is mandatory with 1 deg

Expected behavior
It should handle the case correctly.

To Reproduce

from gammapy.data import DataStore
from gammapy.makers import SafeMaskMaker, MapDatasetMaker
from gammapy.maps import WcsGeom, MapAxis
from gammapy.datasets import MapDataset


data_store = DataStore.from_dir("$GAMMAPY_DATA/hess-dl3-dr1")
selection = dict(
    type="sky_circle",
    frame="icrs",
    lon="83.633 deg",
    lat="22.014 deg",
    radius="5 deg",
)
selected_obs_table = data_store.obs_table.select_observations(selection)
observations = data_store.get_observations(selected_obs_table["OBS_ID"])
energy_axis = MapAxis.from_energy_bounds(1.0, 10.0, 4, unit="TeV")

geom = WcsGeom.create(
    skydir=(85.633, 22.014),
    binsz=0.02,
    width=(10, 10),
    frame="icrs",
    proj="CAR",
    axes=[energy_axis],
)

# Reduced IRFs are defined in true energy (i.e. not measured energy).
energy_axis_true = MapAxis.from_energy_bounds(
    0.5, 20, 10, unit="TeV", name="energy_true"
)

stacked = MapDataset.create(
    geom=geom, energy_axis_true=energy_axis_true, name="crab-stacked"
)
offset_max = 2.5 * u.deg
maker = MapDatasetMaker()
maker_safe_mask = SafeMaskMaker(
    methods=["offset-max", "aeff-max"], fixed_offset="1 deg"
)


for obs in observations:
    empty = stacked.copy(name=f"obs-{obs.obs_id}")
    # A MapDataset is filled in this cutout geometry
    dataset = maker.run(empty, obs)
    # The data quality cut is applied
    dataset = maker_safe_mask.run(dataset, obs)

Triggered by #3978.

[adonath]

The issue here is that the observation argument is not passed correctly to the other methods:

• https://github.com/gammapy/gammapy/blob/master/gammapy/makers/safe.py#L316
• https://github.com/gammapy/gammapy/blob/master/gammapy/makers/safe.py#L319