encoding/wkt: preserve EMPTY members when unmarshalling a collection

[gaoflow]

Problem

wkt.Marshal produces spec-valid WKT for a GEOMETRYCOLLECTION whose members are EMPTY geometries, but wkt.Unmarshal silently drops those members, so the library cannot read back its own output:

gc := orb.Collection{orb.Point{1, 2}, orb.LineString{}}
s := wkt.MarshalString(gc)                 // "GEOMETRYCOLLECTION(POINT(1 2),LINESTRING EMPTY)"
got, _ := wkt.Unmarshal(s)
// got is GEOMETRYCOLLECTION(POINT(1 2)) -- the empty member is gone

This reproduces for every empty-capable member type (LINESTRING/POLYGON/MULTIPOINT/MULTILINESTRING/MULTIPOLYGON/GEOMETRYCOLLECTION EMPTY), for mixed empty/non-empty collections, and for nested collections.

Root cause

splitGeometryCollection only appended a member once a ( had been pushed onto its rune stack. An EMPTY geometry has no parentheses, so its token never triggered a yield and stayed stuck in the stack. unmarshalCollection then discarded the resulting "" via if len(g) == 0 { continue }.

For example splitGeometryCollection("(LINESTRING EMPTY)") returned [""] and "(POINT(1 2),LINESTRING EMPTY)" returned ["", "POINT(1 2)"] -- the empty member vanished.

Fix

Rewrite the splitter to split the collection body on top-level commas while tracking bracket depth. This treats empty members (no parens) and nested-collection members (inner commas) uniformly, and removes the len(g)==0 swallow. Non-empty and nested non-empty collections are unaffected.

Tests

TestUnmarshalCollection_roundTrip asserts the Unmarshal(Marshal(gc)) == gc self-oracle over each empty-capable type, mixed empty/non-empty, and nested collections, checking member count, member equality and marshal idempotence. TestSplitGeometryCollection covers the splitter directly, including nested members whose inner commas must not split. go test ./... passes for the affected packages (the pre-existing simplify Visvalingam benchmark failure is unrelated and also fails on master).