struct GlossaryModel {
    let key: String
    let offences: [Offence]
}

class GlossaryVC: UIViewController, UITableViewDelegate, UITableViewDataSource {

    private var datasource = [GlossaryModel]()
    private var filteredDataSource = [GlossaryModel]()
    override func viewDidAppear(_ animated: Bool) {

        DataProvider.getData(with: .GLOSSARY_DATA_URL) { [weak self] (letters) in
            guard let self = self else { return }
            let allOffences = letters.flatMap { $0.offences }

            //Construct your oldDict, code ~ in groupGlossarys
            let glossarysDictionary: [String: [Offence]] = [:]

            //Then:
            self.datasource = glossarysDictionary.compactMap{
                let sortedOffences = $0.value.sorted(by: { $0.valueForSorting < $1.valueForSorting })
                GlossaryModel(key: $0.key,
                              offences: sortedOffences)
            }
            self.filteredDataSource = self.datasource
        }

    }

    func groupGlossarys() {
         ...
        checkAndShowTableView()
    }

    func checkAndShowTableView() {

        if filteredDataSource.isEmpty == false { //prefers using isEmpty instead of count
            tableView.isHidden = false
            noResultsView.isHidden = true
            tableView.reloadData()
        } else {
            tableView.isHidden = true
            noResultsView.isHidden = false
        }
    }

    func numberOfSections(in tableView: UITableView) -> Int {
        return filteredDataSource.count
    }

    func tableView(_ tableView: UITableView, titleForHeaderInSection section: Int) -> String? {
        return filteredDataSource[section].key
    }

    func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return filteredDataSource[section].offences.count
    }

    func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {

        guard let glossaryCell = tableView.dequeueReusableCell(withIdentifier: glossaryCellId, for: indexPath) as? GlossaryCell else {
            fatalError("Unable to dequeue contact cell")
        }

        let offence = filteredDataSource[indexPath.section].offences[indexPath.row]
        glossaryCell.configure(glossary: offence)
        return glossaryCell
    }
}