After commit #53661 converted the lang-expressions module to using
DoubleValuesSource, we've seen a performance regression for expressions
that use geopoints. Some investigation suggests that this may be due to
GeoLatitudeValueSource and GeoLongitudeValueSource wrapping their
per-document values in a DoubleValues.withDefault() class. Values exposed
via expressions already have a '0' default value, so this extra wrapping is
unnecessary, and is directly on the hot path. This commit removes the extra
wrapping.
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