When the requested end position has no mapping in the source map,
originalPositionFor() returns an object with line/column set to null
rather than omitting them. endLine picked that null up correctly, but
endColumn was computed as `to && to.column + 1`, which due to operator
precedence is `to && (to.column + 1)`, turning null into 1. So callers
could get back { endColumn: 1, endLine: null }, which doesn't match
how every other unmapped case in this function behaves (undefined for
both). Looks like the +1 fix in #2036 covered the mapped case but not
this one.
Guard `to` the same way `from` already is, so an unmapped end position
just stays undefined.
