Fix non-numeric value warning (#627)

* Fix non-numeric value warning

Fixes this warning on generating PDF:
Warning: A non-numeric value encountered in /tcpdf/tcpdf.php on line 5473

* Better fix for non-numeric value warning

Fixes this warning on generating PDF after calling `Text` with a non-numeric value for `$fstroke`:
Warning: A non-numeric value encountered in /tcpdf/tcpdf.php on line 5470

* Update tcpdf.php

Co-authored-by: William Desportes <williamdes@wdes.fr>

---------

Co-authored-by: William Desportes <williamdes@wdes.fr>
Co-authored-by: Nicola Asuni <nicolaasuni@users.noreply.github.com>

tcpdf.php

@@ -22057,7 +22057,7 @@ public function getNumberOfColumns() {
 	public function setTextRenderingMode($stroke=0, $fill=true, $clip=false) {
 		// Ref.: PDF 32000-1:2008 - 9.3.6 Text Rendering Mode
 		// convert text rendering parameters
-		if ($stroke < 0) {
+		if ($stroke < 0 || !is_numeric($stroke)) {
 			$stroke = 0;
 		}
 		if ($fill === true) {