json2geobuf doesn't encode altitude properly if a coordinate is missing altitude value

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Given a MULTIPOINT set like:

```
{
"type": "MultiPoint",
"coordinates": [
[-112.021, 46.58, 1400],
[-112.045, 46.6],
[-112.0331, 46.5901, 1342]
]
}
```
The resulting set, after being converted to a PBF using json2geobuf and decoded with geobuf2json (or other decoders) shows the same (first occurring) altitude for all points in the set:

```
{
"type": "MultiPoint",
"coordinates": [
[-112.021, 46.58, 1400],
[-112.045, 46.6, 1400],
[-112.0331, 46.5901, 1400]
]
}
```
The issue does not occur if the point in the sample set is given an altitude of 0 or another value.

Start by reproducing the reported MULTIPOINT conversion through json2geobuf and geobuf2json using the coordinates shown in the issue. Trace how altitude values are encoded when one coordinate omits altitude, and verify that the missing altitude is not replaced by the first point's value.

Written by the indexing model from the issue text.