refactor: Remove redundant file check from AutoFile shift operators

The shift operators will call the write or read member function, which
already does the check. Also, call sites are free to directly call
::(Un)Serialize(s, obj) to skip this check, so removing it increases
consistency.

Author: MarcoFalke

src/streams.h

@@ -560,15 +560,13 @@ class AutoFile
     template <typename T>
     AutoFile& operator<<(const T& obj)
     {
-        if (!file) throw std::ios_base::failure("AutoFile::operator<<: file handle is nullptr");
         ::Serialize(*this, obj);
         return *this;
     }
 
     template <typename T>
     AutoFile& operator>>(T&& obj)
     {
-        if (!file) throw std::ios_base::failure("AutoFile::operator>>: file handle is nullptr");
         ::Unserialize(*this, obj);
         return *this;
     }
@@ -588,19 +586,13 @@ class CAutoFile : public AutoFile
     template<typename T>
     CAutoFile& operator<<(const T& obj)
     {
-        // Serialize to this stream
-        if (!file)
-            throw std::ios_base::failure("CAutoFile::operator<<: file handle is nullptr");
         ::Serialize(*this, obj);
         return (*this);
     }
 
     template<typename T>
     CAutoFile& operator>>(T&& obj)
     {
-        // Unserialize from this stream
-        if (!file)
-            throw std::ios_base::failure("CAutoFile::operator>>: file handle is nullptr");
         ::Unserialize(*this, obj);
         return (*this);
     }