wgengine/wgcfg: fix validateEndpoints of empty string

Updates tailscale/corp#1238

wgengine/wgcfg/parser_test.go

@@ -53,3 +53,21 @@ func TestParseEndpoint(t *testing.T) {
 		t.Error("Error was expected")
 	}
 }
+
+func TestValidateEndpoints(t *testing.T) {
+	tests := []struct {
+		in   string
+		want error
+	}{
+		{"", nil},
+		{"1.2.3.4:5", nil},
+		{"1.2.3.4:5,6.7.8.9:10", nil},
+		{",", &ParseError{why: "Missing port from endpoint", offender: ""}},
+	}
+	for _, tt := range tests {
+		got := validateEndpoints(tt.in)
+		if !reflect.DeepEqual(got, tt.want) {
+			t.Errorf("%q = %#v (%s); want %#v (%s)", tt.in, got, got, tt.want, tt.want)
+		}
+	}
+}