Mapping table conformance rule throws error when no Mapping table available

## Describe the bug
When there is no mapping table to select, mapping table conformance rule throw error to console and does not update the Conformance Rule name in the selector.

## To Reproduce
Steps to reproduce the behavior OR commands run:
1. Have no mapping table
2. Select Mapping conformance rule under any dataset

## Expected behaviour
Don't try to display mapping tables if there are none.

## Screenshots

## Console
```js
Uncaught TypeError: Cannot read property '_id' of undefined
at ConformanceRuleDialog.beforeShowFragmentChanges (ConformanceRuleDialog.js:259)
at ConformanceRuleDialog.showFormFragment (ConformanceRuleDialog.js:311)
at ConformanceRuleDialog.onRuleSelect (ConformanceRuleDialog.js:179)
at f.b.fireEvent (EventProvider-dbg.js:228)
at f.d.fireEvent (Element-dbg.js:546)
at f.s.fireChange (Select-dbg.js:2144)
at f.s.onSelectionChange (Select-dbg.js:1325)
at f.b.fireEvent (EventProvider-dbg.js:228)
at f.d.fireEvent (Element-dbg.js:546)
at f.fireSelectionChange (ManagedObjectMetadata-dbg.js:782)
```