`bootstrap(reference='climatology')` creates `time` dimension

[aaronspring]

Describe the bug
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Code Sample

result = hindcast.bootstrap(
    iterations=10,
    metric="rmse",
    comparison="e2o",
    dim=[],
    alignment="same_verif",
    reference=["climatology"],
)
print(result)
<xarray.Dataset>
Dimensions:     (time: 61, init: 60, skill: 2, results: 4, lead: 10)
Coordinates:
  * time        (time) object 1955-01-01 00:00:00 ... 2015-01-01 00:00:00
  * init        (init) object 1955-01-01 00:00:00 ... 2014-01-01 00:00:00
  * lead        (lead) int32 1 2 3 4 5 6 7 8 9 10
    valid_time  (init) object nan nan ... 2015-01-01 00:00:00
  * results     (results) <U12 'verify skill' 'p' 'low_ci' 'high_ci'
  * skill       (skill) <U11 'initialized' 'climatology'
Data variables:
    SST         (skill, results, lead, init, time) float64 nan nan ... 0.1272

Expected behavior
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Output of climpred.show_versions()

# Paste the output here climpred.show_versions() here

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