False positives in mapping table checks

[dweindl]

The additional mapping table validation introduced in #480 incorrectly flag annotation-only rows (e.g. https://github.com/PEtab-dev/petab_test_suite/blob/main/petabtests/cases/v2.0.0/sbml/0016/_mapping.tsv) in the following test cases:

Invalid PEtab problem generated for petab_test_suite/petabtests/cases/v2.0.0/sbml/0016.
ERROR: PEtab IDs `['condition1', 'condition2']` are defined in the mapping table but also defined through other PEtab tables. [CheckMappingTable]
Invalid PEtab problem generated for petab_test_suite/petabtests/cases/v2.0.0/sbml/0022.
ERROR: PEtab IDs `['condition2']` are defined in the mapping table but also defined through other PEtab tables. [CheckMappingTable]
Invalid PEtab problem generated for petab_test_suite/petabtests/cases/v2.0.0/sbml/0028.
ERROR: PEtab IDs `['condition1']` are defined in the mapping table but also defined through other PEtab tables. [CheckMappingTable]
Invalid PEtab problem generated for petab_test_suite/petabtests/cases/v2.0.0/sbml/0030.
ERROR: PEtab IDs `['condition1', 'condition2']` are defined in the mapping table but also defined through other PEtab tables. [CheckMappingTable]