optimize capacity & add assert messages in GeoJson

[wjdqhry]

This is a minor improvement; no new tests were added and existing tests pass with these changes.

related spring-projects/spring-data-elasticsearch#3064

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Closes #4904

[christophstrobl]

LGTM

[christophstrobl]

Thank you @wjdqhry for your contribution. Merged to main development line.