Change MythDate::toString() to return an empty string for invalid inp…

…uts.

It's possible a "0000-00-00 00:00:00" might be somewhat acceptable in
some rare cases because the all 0s value obviously represents a
special value.  The UTC to local conversions added in 0.26, however,
now causes it to change to a less obviously special, and perhaps even
misledaing, not all 0s value.  Since MythDate::toString() was a
Myth-specific replacement for QDateTime::toString() and
QDate::toString() and those functions return an empty string for
invalid inputs, we probably should too.

Fixes #11405 .

mythtv/libs/libmythbase/mythdate.cpp

@@ -72,6 +72,9 @@ QString toString(const QDateTime &raw_dt, uint format)
 {
     QString result;
 
+    if (!raw_dt.isValid())
+        return result;
+
     // if no format is set default to UTC for ISO/file/DB dates.
     if (!((format & kOverrideUTC) || (format & kOverrideLocal)))
     {
@@ -118,6 +121,9 @@ QString toString(const QDate &date, uint format)
 {
     QString result;
 
+    if (!date.isValid())
+        return result;
+
     if (format & (kDateFull | kDateShort))
     {
         QString stringformat;