fixup! MCOL-2051: CS uses 'NULL' literal as NULL when UPDATEs varchar field

Fixed to retain compatibility with CentOS 6 (no C++11 support).
Moved initialization to the ColumnAssignment constructor.

dbcon/dmlpackage/dmlpkg.h

@@ -397,6 +397,14 @@ public:
 class ColumnAssignment
 {
 public:
+    explicit ColumnAssignment(
+        std::string const& column,
+        std::string const& op = "=",
+        std::string const& expr = "") :
+        fColumn(column), fOperator(op), fScalarExpression(expr),
+        fFromCol(false), fFuncScale(0), fIsNull(false)
+        {};
+
     /** @brief dump to stdout
      */
     std::ostream& put(std::ostream &os) const;
@@ -411,7 +419,7 @@ public:
     std::string fScalarExpression;
     bool fFromCol;
     uint32_t fFuncScale;
-    bool fIsNull = false;
+    bool fIsNull;
 };
 
 /** @brief Stores a value list or a query specification