Fix some PHP 8.1 warnings (#4012)

* Fix some PHP 8.1 warnings
The proper fix will have to wait till be drop PHP7.
#fix https://github.com/FreshRSS/FreshRSS/issues/4010

* Another PHP8.1 fix

lib/Minz/Pdo.php

@@ -28,6 +28,8 @@ abstract class Minz_Pdo extends PDO {
 		return $this->autoPrefix($statement);
 	}
 
+	// PHP8+: PDO::lastInsertId(?string $name = null): string|false
+	#[\ReturnTypeWillChange]
 	public function lastInsertId($name = null) {
 		if ($name != null) {
 			$name = $this->preSql($name);
@@ -35,16 +37,22 @@ abstract class Minz_Pdo extends PDO {
 		return parent::lastInsertId($name);
 	}
 
+	// PHP8+: PDO::prepare(string $query, array $options = []): PDOStatement|false
+	#[\ReturnTypeWillChange]
 	public function prepare($statement, $driver_options = array()) {
 		$statement = $this->preSql($statement);
 		return parent::prepare($statement, $driver_options);
 	}
 
+	// PHP8+: PDO::exec(string $statement): int|false
+	#[\ReturnTypeWillChange]
 	public function exec($statement) {
 		$statement = $this->preSql($statement);
 		return parent::exec($statement);
 	}
 
+	// PHP8+: PDO::query(string $query, ?int $fetchMode = null, mixed ...$fetchModeArgs): PDOStatement|false
+	#[\ReturnTypeWillChange]
 	public function query($query, $fetch_mode = null, ...$fetch_mode_args) {
 		$query = $this->preSql($query);
 		return $fetch_mode ? parent::query($query, $fetch_mode, ...$fetch_mode_args) : parent::query($query);

lib/Minz/Request.php

@@ -415,7 +415,7 @@ class Minz_Request {
 	 * @return string
 	 */
 	private static function extractContentType() {
-		return strtolower(trim(static::getHeader('CONTENT_TYPE')));
+		return strtolower(trim(static::getHeader('CONTENT_TYPE', '')));
 	}
 
 	/**
@@ -454,7 +454,7 @@ class Minz_Request {
 	 * @return array
 	 */
 	public static function getPreferredLanguages() {
-		if (preg_match_all('/(^|,)\s*(?P<lang>[^;,]+)/', static::getHeader('HTTP_ACCEPT_LANGUAGE'), $matches)) {
+		if (preg_match_all('/(^|,)\s*(?P<lang>[^;,]+)/', static::getHeader('HTTP_ACCEPT_LANGUAGE', ''), $matches)) {
 			return $matches['lang'];
 		}
 		return array('en');

lib/lib_rss.php

@@ -165,7 +165,7 @@ function html_only_entity_decode($text) {
 			get_html_translation_table(HTML_SPECIALCHARS, ENT_NOQUOTES, 'UTF-8')	//Preserve XML entities
 		));
 	}
-	return strtr($text, $htmlEntitiesOnly);
+	return $text == '' ? '' : strtr($text, $htmlEntitiesOnly);
 }
 
 function customSimplePie($attributes = array()) {