Remove some unnecessary returns.

After `throw_null_conversion` (which never returns), some call sites
had a bogus `return` just to shut up compiler warnings.  This was done
a long time ago and it may not matter now.  Visual Studio was issuing
"dead code" warnings for the workarounds.

Fixes #160.

include/pqxx/strconv.hxx

@@ -2,7 +2,7 @@
  *
  * DO NOT INCLUDE THIS FILE DIRECTLY; include pqxx/stringconv instead.
  *
- * Copyright (c) 2008-2018, Jeroen T. Vermeulen.
+ * Copyright (c) 2008-2019, Jeroen T. Vermeulen.
  *
  * See COPYING for copyright license.  If you did not receive a file called
  * COPYING with this source code, please notify the distributor of this mistake,
@@ -190,8 +190,7 @@ template<> struct PQXX_LIBEXPORT string_traits<std::string>
   static constexpr const char *name() noexcept { return "string"; }
   static constexpr bool has_null() noexcept { return false; }
   static bool is_null(const std::string &) { return false; }
-  static std::string null()
-	{ internal::throw_null_conversion(name()); return std::string{}; }
+  static std::string null() { internal::throw_null_conversion(name()); }
   static void from_string(const char Str[], std::string &Obj) { Obj=Str; }
   static std::string to_string(const std::string &Obj) { return Obj; }
 };
@@ -201,8 +200,7 @@ template<> struct PQXX_LIBEXPORT string_traits<const std::string>
   static constexpr const char *name() noexcept { return "const string"; }
   static constexpr bool has_null() noexcept { return false; }
   static bool is_null(const std::string &) { return false; }
-  static const std::string null()
-	{ internal::throw_null_conversion(name()); return std::string{}; }
+  static const std::string null() { internal::throw_null_conversion(name()); }
   static const std::string to_string(const std::string &Obj) { return Obj; }
 };
 
@@ -211,12 +209,7 @@ template<> struct PQXX_LIBEXPORT string_traits<std::stringstream>
   static constexpr const char *name() noexcept { return "stringstream"; }
   static constexpr bool has_null() noexcept { return false; }
   static bool is_null(const std::stringstream &) { return false; }
-  static std::stringstream null()
-  {
-    internal::throw_null_conversion(name());
-    // No, dear compiler, we don't need a return here.
-    throw 0;
-  }
+  static std::stringstream null() { internal::throw_null_conversion(name()); }
   static void from_string(const char Str[], std::stringstream &Obj)
 	{ Obj.clear(); Obj << Str; }
   static std::string to_string(const std::stringstream &Obj)