fix(Bookmark): Remove mb_convert_encoding warning

Signed-off-by: Marcel Klehr <mklehr@gmx.net>

Author: marcelklehr

lib/Db/Bookmark.php

@@ -132,7 +132,7 @@ public function setTextContent(?string $content): void {
 
 	public function getTextContent(): string {
 		// Remove non-utf-8 characters from string: https://stackoverflow.com/questions/1401317/remove-non-utf8-characters-from-string
-		return (string)mb_convert_encoding($this->textContent, 'UTF-8', 'UTF-8');
+		return $this->textContent ? (string)mb_convert_encoding($this->textContent, 'UTF-8', 'UTF-8') : '';
 	}
 
 	public function setHtmlContent(?string $content): void {
@@ -145,7 +145,7 @@ public function setHtmlContent(?string $content): void {
 
 	public function getHtmlContent(): string {
 		// Remove non-utf-8 characters from string: https://stackoverflow.com/questions/1401317/remove-non-utf8-characters-from-string
-		return (string)mb_convert_encoding($this->htmlContent, 'UTF-8', 'UTF-8');
+		return $this->htmlContent ? (string)mb_convert_encoding($this->htmlContent, 'UTF-8', 'UTF-8') : '';
 	}
 
 	public function isWebLink() {