MCOL-271 empty strings should not be NULLs (#2794)

This patch improves handling of NULLs in textual fields in ColumnStore.
Previously empty strings were considered NULLs and it could be a problem
if data scheme allows for empty strings. It was also one of major
reasons of behavior difference between ColumnStore and other engines in
MariaDB family.

Also, this patch fixes some other bugs and incorrect behavior, for
example, incorrect comparison for "column <= ''" which evaluates to
constant True for all purposes before this patch.

utils/messageqcpp/bytestream.h

@@ -36,6 +36,7 @@
 #include "exceptclasses.h"
 #include "serializeable.h"
 #include "any.hpp"
+#include "nullstring.h"
 
 class ByteStreamTestSuite;
 
@@ -158,6 +159,10 @@ class ByteStream : public Serializeable
    * push a std::string onto the end of the stream.
    */
   EXPORT ByteStream& operator<<(const std::string& s);
+  /**
+   * push a NullString onto the end of the stream.
+   */
+  EXPORT ByteStream& operator<<(const utils::NullString& s);
   /**
    * push an arbitrary class onto the end of the stream.
    */
@@ -231,6 +236,10 @@ class ByteStream : public Serializeable
    * extract a std::string from the front of the stream.
    */
   EXPORT ByteStream& operator>>(std::string& s);
+  /**
+   * extract a NullString from the front of the stream.
+   */
+  EXPORT ByteStream& operator>>(utils::NullString& s);
   /**
    *	write the current stream into b. The ByteStream will be empty after this operation.
    * @warning the caller is responsible for making sure b is big enough to hold all the data (perhaps by