Catch and fix uninitialized QStrings passed to SQL code.

Most of Qt5 treats an uninitialized string and an empty string as the
same thing.  QSqlQuery doesn't, storing NULL for the former and "" for
the latter.  Add a new function for storing a non-null database field
that will store both of these QStrings as "".

Fixes #13420

mythtv/libs/libmythbase/mythdbcon.cpp

@@ -866,6 +866,16 @@ void MSqlQuery::bindValue(const QString &placeholder, const QVariant &val)
     QSqlQuery::bindValue(placeholder, val, QSql::In);
 }
 
+void MSqlQuery::bindValueNoNull(const QString &placeholder, const QVariant &val)
+{
+    if ((val.type() == QVariant::String) && val.isNull())
+    {
+        QSqlQuery::bindValue(placeholder, QString(""), QSql::In);
+        return;
+    }
+    QSqlQuery::bindValue(placeholder, val, QSql::In);
+}
+
 void MSqlQuery::bindValues(const MSqlBindings &bindings)
 {
     MSqlBindings::const_iterator it;

mythtv/libs/libmythbase/mythdbcon.h

@@ -159,8 +159,24 @@ class MBASE_PUBLIC MSqlQuery : private QSqlQuery
     /// \brief QSqlQuery::prepare() is not thread safe in Qt <= 3.3.2
     bool prepare(const QString &query);
 
+    /// \brief Add a single binding
     void bindValue(const QString &placeholder, const QVariant &val);
 
+    /// \brief Add a single binding, taking care not to set a NULL value.
+    ///
+    /// Most of Qt5 treats an uninitialized (i.e. null) string and an
+    /// empty string as the same thing, but not the QSqlQuery code.
+    /// This means that an uninitialized QString() and an explicitly
+    /// initialized to empty QString("") both represent an empty
+    /// string everywhere in Qt except in the QSqlQuery code.  This
+    /// function adds the same behavior to SQL queries, by checking
+    /// and substituting a QString("") where necessary.  This function
+    /// should be used for any database string field that has been
+    /// declared "NOT NULL" so that MythTV won't throw an errors
+    /// complaining about trying to set a NULL value in a SQL column
+    /// that's marked "non-NULL".
+    void bindValueNoNull(const QString &placeholder, const QVariant &val);
+
     /// \brief Add all the bindings in the passed in bindings
     void bindValues(const MSqlBindings &bindings);
 

mythtv/libs/libmythtv/channelinfo.cpp

@@ -323,26 +323,26 @@ bool ChannelInsertInfo::SaveScan(uint scanid, uint transportid) const
     query.bindValue(":MPLEX_ID", m_db_mplexid);
     query.bindValue(":SOURCE_ID", m_source_id);
     query.bindValue(":CHANNEL_ID", m_channel_id);
-    query.bindValue(":CALLSIGN", m_callsign);
-    query.bindValue(":SERVICE_NAME", m_service_name);
-    query.bindValue(":CHAN_NUM", m_chan_num);
+    query.bindValueNoNull(":CALLSIGN", m_callsign);
+    query.bindValueNoNull(":SERVICE_NAME", m_service_name);
+    query.bindValueNoNull(":CHAN_NUM", m_chan_num);
     query.bindValue(":SERVICE_ID", m_service_id);
     query.bindValue(":ATSC_MAJOR_CHANNEL", m_atsc_major_channel);
     query.bindValue(":ATSC_MINOR_CHANNEL", m_atsc_minor_channel);
     query.bindValue(":USE_ON_AIR_GUIDE", m_use_on_air_guide);
     query.bindValue(":HIDDEN", m_hidden);
     query.bindValue(":HIDDEN_IN_GUIDE", m_hidden_in_guide);
-    query.bindValue(":FREQID", m_freqid);
-    query.bindValue(":ICON", m_icon);
-    query.bindValue(":TVFORMAT", m_format);
-    query.bindValue(":XMLTVID", m_xmltvid);
+    query.bindValueNoNull(":FREQID", m_freqid);
+    query.bindValueNoNull(":ICON", m_icon);
+    query.bindValueNoNull(":TVFORMAT", m_format);
+    query.bindValueNoNull(":XMLTVID", m_xmltvid);
     query.bindValue(":PAT_TSID", m_pat_tsid);
     query.bindValue(":VCT_TSID", m_vct_tsid);
     query.bindValue(":VCT_CHAN_TSID", m_vct_chan_tsid);
     query.bindValue(":SDT_TSID", m_sdt_tsid);
     query.bindValue(":ORIG_NETID",  m_orig_netid);
     query.bindValue(":NETID", m_netid);
-    query.bindValue(":SI_STANDARD", m_si_standard);
+    query.bindValueNoNull(":SI_STANDARD", m_si_standard);
     query.bindValue(":IN_CHANNELS_CONF", m_in_channels_conf);
     query.bindValue(":IN_PAT", m_in_pat);
     query.bindValue(":IN_PMT", m_in_pmt);
@@ -355,7 +355,7 @@ bool ChannelInsertInfo::SaveScan(uint scanid, uint transportid) const
     query.bindValue(":IS_OPEBCABLE", m_is_opencable);
     query.bindValue(":COULD_BE_OPENCABLE", m_could_be_opencable);
     query.bindValue(":DECRYPTION_STATUS", m_decryption_status);
-    query.bindValue(":DEFAULT_AUTHORITY", m_default_authority);
+    query.bindValueNoNull(":DEFAULT_AUTHORITY", m_default_authority);
 
     if (!query.exec())
     {