Throw Error when finding an empty XML tag

the benchmark /ogs_source/Tests/Data//TH2M/THM/slab/THM_1d_dirichlet.prj has an empty process_variables/process_variable/source_terms for the "Temperatur equation" field like this:

<source_terms>

</source_terms>

It is read in as "character(0)" and is not recognized as "null" as it would be if the node was missing altogether. The best would be if en error is thrown when an empty tag is encountered.