tidy: Explicitly check file size in previewgenerator.cpp.

The clang-tidy "implicit boolean conversion" check pointed out a spot
where a file size was implicitly converted to a boolean value.  Fix to
explicitly test to see if the file is non-zero in length.  Changes
made by the clang-tidy program.

https://clang.llvm.org/extra/clang-tidy/checks/readability-implicit-bool-conversion.html

mythtv/libs/libmythtv/previewgenerator.cpp

@@ -292,7 +292,7 @@ bool PreviewGenerator::Run(void)
             }
 
             QFileInfo fi(outname);
-            ok = (fi.exists() && fi.isReadable() && fi.size());
+            ok = (fi.exists() && fi.isReadable() && (fi.size() != 0));
             if (ok)
             {
                 LOG(VB_PLAYBACK, LOG_INFO, LOC + "Preview process ran ok.");