Using callback function to retrieve NULL values in C++ / SQLite - c++

Question

I am using C++ and SQLite for a basic messageboard system. I have Datetime fields in my Messages table called 'posted' for the original post date, and 'edited' for the edited date, which is NULL if the message has not been edited. I can post messages successfully but I am running into problems when trying to display posted messages with a NULL edited value.
static int getData(const char* s)
{
sqlite3* DB;
int exit = sqlite3_open(s, &DB);
string sql = "SELECT MSGID, POSTED, EDITED, CONTENT FROM MESSAGES WHERE USERID IS '123' ";
"ORDER BY DATETIME(POSTED) DESC;";
sqlite3_exec(DB, sql.c_str(), callback, NULL, NULL);
sqlite3_close(DB);
return 0;
}
static int callback(void* NotUsed, int argc, char** argv, char** azColName)
{
for (int i = 0; i < argc; i++) {
cout << azColName[i] << ": " << argv[i] << endl;
}
return 0;
}
In the console this returns:
MSGID: 1
POSTED: 2020-03-27 12:14:59
EDITED:
Then diverts to the end of the program without displaying the content value.
I am setting EDITED to NULL when posting a message, and I've also tried leaving out setting the value at all, and entering an empty string, but I'm getting the same problem. If I remove EDITED from the SQL statement, it continues to the end and shows the CONTENT value.
static int getData(const char* s)
{
sqlite3* DB;
int exit = sqlite3_open(s, &DB);
string sql = "SELECT MSGID, POSTED, CONTENT FROM MESSAGES WHERE USERID IS '123' ";
"ORDER BY DATETIME(POSTED) DESC;";
sqlite3_exec(DB, sql.c_str(), callback, NULL, NULL);
sqlite3_close(DB);
return 0;
}
static int callback(void* NotUsed, int argc, char** argv, char** azColName)
{
for (int i = 0; i < argc; i++) {
cout << azColName[i] << ": " << argv[i] << endl;
}
return 0;
}
returns
MSGID: 1
POSTED: 2020-03-27 12:14:59
CONTENT: This is a message
so I assume it is something to do with how the callback function handles the NULL value, but I'm struggling to find guidance on how to display the NULL values and continue with the rest of the For loop.
Thanks!
EDIT:
Following on from Paul Sanders' prompt, I've amended the callback function to the following, which is displaying how I wanted it to:
static int callbackMsg(void* NotUsed, int argc, char** argv, char** azColName)
{
for (int i = 0; i < argc; i++) {
cout << azColName[i] << ": ";
if (argv[i]) {
cout << argv[i] << endl;
}
else
cout << endl;
continue;
}
cout << endl;
return 0;
}
MSGID: 1
POSTED: 2020-03-27 12:14:59
EDITED:
CONTENT: This is a message