BUG: fix a spurious warning when calling sigma_clipped_stats on a MaskedColumn

[neutrinoceros]

Description

Fixes #13281
Patch initially proposed by @mhvk

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[mhvk]

Thanks! I might have checked that the output is identical myself, but, really, it is. So, approving (but won't merge yet to give @larrybradley a chance to have a look too).

[larrybradley]

Thanks!