Return empty string from wxColour::GetAsString() if it's invalid

This is consistent with wxToString(wxColour) and seems more useful than
either returning black string representation (as wxMSW used to do) or
asserting (as wxGTK did).

Document this behaviour and add a test checking for it.

Closes #18623.

tests/graphics/colour.cpp

@@ -145,6 +145,16 @@ TEST_CASE("wxColour::FromString", "[colour][string]")
     CHECK( !wxFromString("rgba(1, 2, 3.456, foo)", &col) );
 }
 
+TEST_CASE("wxColour::GetAsString", "[colour][string]")
+{
+    CHECK( wxColour().GetAsString() == "" );
+
+    wxColour red("red");
+    CHECK( red.GetAsString() == "red" );
+    CHECK( red.GetAsString(wxC2S_CSS_SYNTAX) == "rgb(255, 0, 0)" );
+    CHECK( red.GetAsString(wxC2S_HTML_SYNTAX) == "#FF0000" );
+}
+
 TEST_CASE("wxColour::GetLuminance", "[colour][luminance]")
 {
     CHECK( wxBLACK->GetLuminance() == Approx(0.0) );