fix: MCOL-5755: incorrect handling of BLOB (and TEXT) in GROUP BY

BLOB fields did not work as grouping keys at all, they were assigned
value NULL for any value, be it NULL or not. The fix is in the
rowaggregation.cpp in the initMapping(), a switch/case branch was added
to handle BLOB field copying there.

Also, TEXT columns did not distinguish between NULL and empty string in
the grouping algorithm, now they do. The fix is in the equals()
function, now we specifically check for isNull() equality between
values.

utils/rowgroup/rowgroup.h

@@ -562,6 +562,9 @@ class Row
   inline void setBinaryField(const T* value, uint32_t colIndex);
   template <typename T>
   inline void setBinaryField_offset(const T* value, uint32_t width, uint32_t colIndex);
+  // XXX: TODO: I'd deprecate these two functions in favor of get/setStringField.
+  // getSetStringField properly support binary data of up to 4G bytes
+  // and also provide perfomant interface through use of ConstString.
   // support VARBINARY
   // Add 2-byte length at the CHARSET_INFO*beginning of the field.  nullptr and zero length field are
   // treated the same, could use one of the length bit to distinguish these two cases.