fix: issue with GetHeader called with non-existing header

[JohnVillalovos]

This would cause errors like:
PHP Warning: Undefined array key "HTTP_ACCEPT_LANGUAGE" in
/home/runner/work/librebooking/librebooking/lib/Server/Server.php on
line 272

PHP Deprecated: strlen(): Passing null to parameter #1 ($string) of
type string is deprecated in
/home/runner/work/librebooking/librebooking/lib/Server/Server.php on
line 260

[Copilot]

Pull request overview

This PR fixes PHP warnings that occur when GetHeader is called with a non-existing header key. The warnings include an "Undefined array key" error and a deprecated null parameter warning for strlen(). The fix adds a null coalescing operator to return an empty string when the header doesn't exist.

Changes:

• Modified GetHeader method in lib/Server/Server.php to return an empty string instead of undefined when a header key doesn't exist in $_SERVER

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[Copilot]

This change returns an empty string for missing headers, which is inconsistent with other similar methods in the Server class. Methods like GetCookie, GetQuerystring, GetRawForm, and GetSession all return null for missing values.

Additionally, this is inconsistent with the IRestServer interface (lib/WebService/IRestServer.php:43), which defines GetHeader as returning string|null. While Server.php doesn't implement IRestServer, the inconsistency could cause confusion.

Consider using return $_SERVER[$headerCode] ?? null; instead to maintain consistency with the codebase pattern and match the interface definition.

Note: The docblock on line 268 also claims "@return string" but should be "@return string|null" to reflect the actual behavior.