graph/db: fix DecodeHexColor

Ensure that it does a sanity check on the length of its input and also
only call it if the nullable SQL string is not null.

Expand an existing unit tests to cover the DecodeHexColor such that it
would have caught the bug.

graph/notifications_test.go

@@ -991,7 +991,9 @@ func TestChannelCloseNotification(t *testing.T) {
 // TestEncodeHexColor tests that the string used to represent a node color is
 // correctly encoded.
 func TestEncodeHexColor(t *testing.T) {
-	var colorTestCases = []struct {
+	t.Parallel()
+
+	var tests = []struct {
 		R       uint8
 		G       uint8
 		B       uint8
@@ -1006,14 +1008,32 @@ func TestEncodeHexColor(t *testing.T) {
 		{1, 2, 3, "#", false},
 	}
 
-	for _, tc := range colorTestCases {
-		encoded := graphdb.EncodeHexColor(
-			color.RGBA{tc.R, tc.G, tc.B, 0},
+	for _, test := range tests {
+		t.Run(fmt.Sprintf("R:%d,G:%d,B:%d", test.R, test.G, test.B),
+			func(t *testing.T) {
+				expColor := color.RGBA{
+					R: test.R,
+					G: test.G,
+					B: test.B,
+				}
+
+				encoded := graphdb.EncodeHexColor(expColor)
+
+				if test.isValid {
+					require.Equal(t, test.encoded, encoded)
+				}
+
+				decoded, err := graphdb.DecodeHexColor(
+					test.encoded,
+				)
+				if test.isValid {
+					require.NoError(t, err)
+					require.Equal(t, expColor, decoded)
+				} else {
+					require.Error(t, err)
+				}
+			},
 		)
-		if (encoded == tc.encoded) != tc.isValid {
-			t.Fatalf("incorrect color encoding, "+
-				"want: %v, got: %v", tc.encoded, encoded)
-		}
 	}
 }