Add null check to cope with missing Content-Type header

[sbholmes]

I started getting NPEs when adding my Sony TV, although it had been working fine previously - not sure what changed. The below null check fixes it.

As you can see from these logs, it's the call to get the file sony/ircc/IRCCSCPD.xml that results in a response with no Content-Type header.

2024-11-02 00:21:40.754 [INFO ] [g.sony.internal.AbstractThingHandler] - Thing initialization is called, trying to connect...
2024-11-02 00:21:40.914 [INFO ] [inding.sony.internal.net.HttpRequest] - request for http://TV_IP:37774/sony/webapi/ssdp/dd.xml, response headers {accept-range=[none], connection=[close], content=[text/xml; charset=utf-8], Content-Length=[5534], date=[Fri, 01 Nov 2024 11:21:12 GMT], server=[FedoraCore/2 UPnP/1.0 MINT-X/1.8.1], Content-Type=[text/xml; charset="utf-8"]}
2024-11-02 00:21:41.636 [INFO ] [inding.sony.internal.net.HttpRequest] - request for http://172.23.50.37:37774/sony/webapi/ssdp/dd.xml, response headers {accept-range=[none], connection=[close], content=[text/xml; charset=utf-8], Content-Length=[5534], date=[Fri, 01 Nov 2024 11:21:13 GMT], server=[FedoraCore/2 UPnP/1.0 MINT-X/1.8.1], Content-Type=[text/xml; charset="utf-8"]}
2024-11-02 00:21:41.702 [INFO ] [inding.sony.internal.net.HttpRequest] - request for http://TV_IP:37774/sony/webapi/ssdp/scpd/WebApiSCPD.xml, response headers {accept-range=[none], connection=[close], content=[text/xml; charset=utf-8], Content-Length=[323], date=[Fri, 01 Nov 2024 11:21:13 GMT], last-modified=[Tue, 29 Oct 2024 23:08:28 GMT], server=[FedoraCore/2 UPnP/1.0 MINT-X/1.8.1], Content-Type=[text/xml; charset="utf-8"]}
2024-11-02 00:21:41.759 [INFO ] [inding.sony.internal.net.HttpRequest] - request for http://TV_IP/sony/ircc/IRCCSCPD.xml, response headers {connection=[keep-alive], transfer-encoding=[chunked]}
2024-11-02 00:21:42.321 [INFO ] [inding.sony.internal.net.HttpRequest] - request for http://TV_IP:37774/sony/webapi/ssdp/dd.xml, response headers {accept-range=[none], connection=[close], content=[text/xml; charset=utf-8], Content-Length=[5534], date=[Fri, 01 Nov 2024 11:21:13 GMT], server=[FedoraCore/2 UPnP/1.0 MINT-X/1.8.1], Content-Type=[text/xml; charset="utf-8"]}
2024-11-02 00:21:42.334 [INFO ] [inding.sony.internal.net.HttpRequest] - request for http://TV_IP:37774/sony/webapi/ssdp/scpd/WebApiSCPD.xml, response headers {accept-range=[none], connection=[close], content=[text/xml; charset=utf-8], Content-Length=[323], date=[Fri, 01 Nov 2024 11:21:13 GMT], last-modified=[Tue, 29 Oct 2024 23:08:28 GMT], server=[FedoraCore/2 UPnP/1.0 MINT-X/1.8.1], Content-Type=[text/xml; charset="utf-8"]}
2024-11-02 00:21:42.350 [INFO ] [inding.sony.internal.net.HttpRequest] - request for http://TV_IP/sony/ircc/IRCCSCPD.xml, response headers {connection=[keep-alive], transfer-encoding=[chunked]}
2024-11-02 00:21:46.782 [INFO ] [g.sony.internal.AbstractThingHandler] - Starting state polling every 30 seconds