On the network variables page, the harmonisation matrix is displayed even though the selected topic and cohort have no mappings.

[hslh]

What version of EMX2 are you using (see footer)
v11.20.0

Describe the bug
On the network variables page, the harmonisation matrix is displayed even though the selected topic and cohort have no mappings.

To Reproduce
Steps to reproduce the behavior:

1. Go to 'https://emx2.dev.molgenis.org/catalogue-demo/ssr-catalogue/testNetwork1/variables'
2. Filter on topic 'Language'
3. Filter on source 'testCohort2'
4. Click Harmonisations button
5. See empty matrix is displayed

Expected behavior
No harmonisation matrix is shown, just the message 'No variables found with current filters.'

Desktop (please complete the following information):

• OS: MacOS Sequoia, Firefox v131

Additional context
Future work: On the network variables page, the harmonisation matrix is displayed even though the selected topic and cohort have no mappings. See: /ssr-catalogue/testNetwork1/variables?page=1&conditions=[{"id":"topics","conditions":[{"name":"Language"},{"name":"Socio-demographic+and+economic+characteristics"}]},{"id":"collections","conditions":[{"name":"testCohort2"}]}]&view=harmonisation . (test 7 step 40, 46)
EDIT: in test plan 7 step 38 we just have the 'language' filter active --> 4 variables, all cohorts shown, testcohort2 has no mappings at all. Add the filter on testcohort2 --> randomly 1 variable (testVarNoRepeats) is shown (and not all 4), while the list should be empty. And in test plan 7 step 46 we have no filters active other than 1 cohort, testcohort2. 0 variables should be displayed but we get 2 seemingly random variables displayed, with no mappings (testVarNoRepeats and testVarRepeats_).

[hslh]

Cannot reproduce on acc (v11.20.6) nor on dev (v11.20.9), perhaps #4349 fixed this as well.