Fix Issue #18070 - vtkFLUENTReader

2 issues has been resolved in vtkFLUENTReader (see Issue #18070)

1. Data Array Selection works now as you can select desired variables when reading Fluent file.
2. Empty variable (variable not found due to recent version of Fluent format changes) are replaced by a specific name "VAR_NOT_FOUND_#" instead of an empty string "".

There is still one issue pending from #18070. If the Data Array selection menu is updated in Paraview (after the first reading), nothing changes in the variable menu...

Cc: @cory.quammen