cmd/cloner: preserve nil-valued entries when cloning map (#19749)

The codegen path for map-of-slice-of-pointer fields, skipped
nil-valued entries. That dropped the key from the map.

This broke how dns.Config.Routes uses nil values sentinels.

Fixes #19730
Fixes #19732
Fixes #19746
Fixes #19744

Change-Id: Ic6400227f4ab21b3ca0e8c0eeecf9b83d145a9ab

Signed-off-by: Fernando Serboncini <fserb@tailscale.com>

cmd/cloner/cloner.go

@@ -176,6 +176,7 @@ func gen(buf *bytes.Buffer, it *codegen.ImportTracker, typ *types.Named) {
 				if codegen.ContainsPointers(sliceType.Elem()) {
 					writef("\tfor k, sv := range src.%s {", fname)
 					writef("\t\tif sv == nil {")
+					writef("\t\t\tdst.%s[k] = nil", fname)
 					writef("\t\t\tcontinue")
 					writef("\t\t}")
 					writef("\t\tdst.%s[k] = make([]%s, len(sv))", fname, n)