collections: Don't panic when Has called on uninitialized Set

The convention for Go maps is that read operations treat an uninitialized
map as if empty, and collections.Set is intended to behave in a map-like
way, so Set.Has should return false when called on a totally-uninitialized
set.

Previously this would panic because an uninitialized set has a nil key
function.

internal/collections/set.go

@@ -49,6 +49,11 @@ func NewSetCmp[T comparable]() Set[T] {
 // Has returns true if the given value is present in the set, or false
 // otherwise.
 func (s Set[T]) Has(v T) bool {
+	if len(s.members) == 0 {
+		// We'll skip calling "s.key" in this case, so that we don't panic
+		// if called on an uninitialized Set.
+		return false
+	}
 	k := s.key(v)
 	_, ok := s.members[k]
 	return ok

internal/collections/set_test.go

@@ -37,3 +37,17 @@ func TestSet(t *testing.T) {
 		t.Errorf("set doesn't have \"b\" after adding it")
 	}
 }
+
+func TestSetUninit(t *testing.T) {
+	// An zero-value set should behave like it's empty for read-only operations.
+	var zeroSet Set[string]
+	if got, want := zeroSet.Len(), 0; got != want {
+		t.Errorf("wrong number of elements\ngot:  %d\nwant: %d", got, want)
+	}
+	if zeroSet.Has("anything") {
+		// (this is really just testing that we can call Has without panicking;
+		// it's unlikely that this would ever fail by successfully lying about
+		// a particular member being present.)
+		t.Error("Has reported that \"anything\" is present")
+	}
+}