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.

writeengine/shared/we_type.h

@@ -39,6 +39,7 @@
 #include "calpontsystemcatalog.h"
 #include "IDBDataFile.h"
 #include "IDBPolicy.h"
+#include "nullstring.h"
 
 #undef EXPORT
 #undef DELETE
@@ -331,7 +332,7 @@ typedef std::vector<ColTupleList> ColValueList;                            /** @
 typedef std::vector<RID> RIDList;                                          /** @brief RID list */
 typedef std::vector<execplan::CalpontSystemCatalog::ColType> CSCTypesList; /** @brief CSC column types list */
 
-typedef std::vector<std::string> dictStr;
+typedef std::vector<NullString> dictStr;
 typedef std::vector<dictStr> DictStrList;
 
 // dictionary
@@ -408,7 +409,7 @@ struct JobColumn /** @brief Job Column Structure */
   unsigned long long fDefaultUInt; /** @brief UnsignedInt col default*/
   double fDefaultDbl;              /** @brief Dbl/Flt column default */
   int128_t fDefaultWideDecimal;    /** @brief Wide decimal column default */
-  std::string fDefaultChr;         /** @brief Char column default */
+  utils::NullString fDefaultChr;   /** @brief Char column default */
   JobColumn()
    : mapOid(0)
    , dataType(execplan::CalpontSystemCatalog::INT)