Support tables without input clauses

[nikku]

The DMN standard mandates, that there can also be zero input clauses. dmn-js though automatically adds the input clause and makes it impossible for users to model tables without input clauses.

Tasks

• [x] Be able to import tables without input clauses
• [x] Be able to delete all input clauses
• [x] Generate decision tables with an input clause per default

[nikku]

Until this issue is addressed we'll fail with an error whenever a decision table without inputs or outputs is imported.