LibJS: Change RelativeTimeFormat::number_format to return a reference

In the one place this will be used, we will know that the NumberFormat
object is non-null. So return a reference, as the AO it is passed off to
also expects a reference.

Userland/Libraries/LibJS/Runtime/Intl/RelativeTimeFormat.h

@@ -51,7 +51,7 @@ public:
     void set_numeric(StringView numeric);
     StringView numeric_string() const;
 
-    NumberFormat* number_format() const { return m_number_format; }
+    NumberFormat& number_format() const { return *m_number_format; }
     void set_number_format(NumberFormat* number_format) { m_number_format = number_format; }
 
 private: