BUG: volFieldValue: writes empty fields

Summary

volFieldValue FO writes empty fields when the input entry is writeFields=true.

It is due to the weightField being empty when there are no weights, which caused the entire output to be empty instead of simply being unweighted.

Steps to reproduce

• Download/extract cavity.zip
• ./Allrun
• Inspect p_all in the time directories.

What is the expected correct behavior?

volFieldValue FO should write non-empty fields.

Environment information

• OpenFOAM version : 18c68e6b