errors when running mythfilldatabase with Qt6 and NOT NULL database fields

[garybuhrmaster]

Platform: Fedora 41, building/running with Qt6 6.8.0.

There are some cases in Fedora 41 with Qt6 in which showtype, colorcode, and title_pronounce are used as a null rather than an empty string, resulting in errors when running mythfilldatabase of the form:

Original query failed, but resend with empty strings in place of NULL strings worked.
DB Error (MSqlQuery):
Query was:
REPLACE INTO program (  chanid,         title,          subtitle,        description,   category,       category_type,    starttime,      endtime,   closecaptioned, stereo,         hdtv,            subtitled,   subtitletypes,  audioprop,      video>
Driver error was [2/1048]:
QMYSQL: Unable to execute statement
Database error was:
Column 'showtype' cannot be null

The showtype, colorcode, and title_pronounce columns are defined as NOT NULL, so it would seem that these should always have been surrounded with a denullify().  While for Qt5 null values are converted to empty strings, for Qt6 it must be explicit. In the case of inetref, while it is defined as possible being NULL, the Qt5 implementations stored it as empty, so match that behavior in Qt6 (as the expression "= NULL" is not the same as "is NULL").

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PR will be created with a proposed fix