Fix CHT boundary problem for flamelet computations

[Cristopher-Morales]

Proposed Changes

Continuation of previous pull request #2281

[CHT + flamelet was only working if there was an additional isothermal wall present.]

Related Work

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[bigfooted]

To me it looks like a better solution compared to the previous one because it prevents going into the isothermal wall function unnecessarily. LGTM, but maybe @pcarruscag can have a second look?