DynamicAny non-initialized state capability

made DynamicAnyHolder::create() and is...() pure virtual functions virtual
(simplifies DynamicAnyHolderImpl implementation)
made DynamicAnyHolder default constructor protected
made DynamicAnyHolderImpl default constructor private
added DynamicAnyTest::testEmpty()

Data/include/Poco/Data/BLOB.h

@@ -373,6 +373,7 @@ public:
 	}
 
 private:
+	DynamicAnyHolderImpl();
 	Data::BLOB _val;
 };
 

Data/include/Poco/Data/Date.h

@@ -306,6 +306,7 @@ public:
 	}
 
 private:
+	DynamicAnyHolderImpl();
 	Data::Date _val;
 };
 

Data/include/Poco/Data/Time.h

@@ -310,6 +310,7 @@ public:
 	}
 
 private:
+	DynamicAnyHolderImpl();
 	Data::Time _val;
 };