Fix ocean multimap

[zklaus]

Description

• Closes recipe_ocean_multimap.yml crashes with regular grid data #2398
• Link to documentation:

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[zklaus]

@esmvalbot please run recipe_ocean_multimap.yml

[esmvalbot]

Since @zklaus asked, ESMValBot will run recipe recipe_ocean_multimap.yml as soon as possible, output will be generated here

[esmvalbot]

ESMValBot is happy to report recipe recipe_ocean_multimap.yml ran OK, output has been generated here