feat(update): allow clearing values with "" and null on update

Update handlers skipped empty strings and nulls, so a client could not
blank or null out a column. Every key present in the payload now
overwrites the stored value, including "" and null.

- add common.MergeUpdateValues and use it in resolvespec and restheadspec
- add Handler.SetDisallowNulls to skip null values (""still overwrites)
- websocketspec and mqttspec now write only the keys present in the
  payload via SetMap instead of updating the whole zeroed model, which
  clobbered absent fields

pkg/common/update_merge_test.go

@@ -0,0 +1,29 @@
+package common
+
+import "testing"
+
+func TestMergeUpdateValues(t *testing.T) {
+	newExisting := func() map[string]interface{} {
+		return map[string]interface{}{"name": "old", "note": "keep", "other": "x"}
+	}
+	incoming := map[string]interface{}{"name": "", "note": nil}
+
+	got := MergeUpdateValues(newExisting(), incoming, false)
+	if got["name"] != "" {
+		t.Errorf("name = %v, want empty string", got["name"])
+	}
+	if v, ok := got["note"]; !ok || v != nil {
+		t.Errorf("note = %v (present=%v), want nil", v, ok)
+	}
+	if got["other"] != "x" {
+		t.Errorf("absent key changed: %v", got["other"])
+	}
+
+	got = MergeUpdateValues(newExisting(), incoming, true)
+	if got["name"] != "" {
+		t.Errorf("disallowNulls: name = %v, want empty string", got["name"])
+	}
+	if got["note"] != "keep" {
+		t.Errorf("disallowNulls: note = %v, want keep", got["note"])
+	}
+}