skip simple procs if their input is not present

[FynnBe]

allows for pre- and postprocessing of optional inputs

Fixes #386

[FynnBe]

tested with

from pathlib import Path

import bioimageio.core
from bioimageio.core.digest_spec import get_test_inputs
from bioimageio.spec import save_bioimageio_package_as_folder
from bioimageio.spec.model import v0_5

local = save_bioimageio_package_as_folder(
    "diplomatic-bug/staged/1", output_path=Path("diplomatic-bug-local")
)
# local = "diplomatic-bug-local"
model_description = bioimageio.core.load_description(local)
assert isinstance(model_description, v0_5.ModelDescr)
inputs = get_test_inputs(model_description)
with bioimageio.core.create_prediction_pipeline(model_description) as pp:
    del inputs.members[bioimageio.core.MemberId("point_prompts")]
    prediction = pp.predict_sample_without_blocking(inputs)