Undefined array key "None" in tcpdf_colors.php

[KasparAas]

Description:
I’ve encountered a warning in tcpdf_colors.php at line 260 when providing an incorrect css attribute "border: 0 #ffffff none;". The warning arises from a case where a specified spot color does not exist or incorrect:
return $spotc[self::$spotcolor[$color][4]];

Issue:
When an undefined color key is accessed, PHP displays a warning: Undefined array key "None". This can happen if the spot color is not set or does not exist in the $spotc array. This will lead to another error if we still continue generating a pdf: "TCPDF ERROR: Some data has already been output, can't send PDF file"

Proposed Solution:
To handle this situation more effectively, I suggest either throwing an appropriate exception to clearly indicate the missing color or using false as a fallback value, which would prevent warnings and ensure smoother handling:
return $spotc[self::$spotcolor[$color][4]] ?? false;

This adjustment would provide more reliable behavior in cases where a color key is undefined.