Missing tranformedGroups causes error

[fschrempf]

Currently it's not possible to have a config without or with empty tranformedGroups, even if no translations between CT and LDAP groups are required.

[16:06:03.852] ERROR (7): Cannot read properties of undefined (reading 'forEach')
    err: {
      "type": "TypeError",
      "message": "Cannot read properties of undefined (reading 'forEach')",
      "stack":
          TypeError: Cannot read properties of undefined (reading 'forEach')
              at exports.getChurchToolsData (/ldap/src/ctservice.js:106:20)
              at initCache (/ldap/src/main.js:20:33)
              at exports.start (/ldap/src/main.js:73:34)
              at run (/ldap/index.js:78:24)
              at Object.<anonymous> (/ldap/index.js:89:1)
              at Module._compile (node:internal/modules/cjs/loader:1119:14)
              at Module._extensions..js (node:internal/modules/cjs/loader:1173:10)
              at Module.load (node:internal/modules/cjs/loader:997:32)
              at Module._load (node:internal/modules/cjs/loader:838:12)
              at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:81:12)
    }