"Update variable" feature in flowable-admin doesn't work if value is null

**Describe the bug**
In the flowable-admin app:
* Go to "Process Engine" > "Instances"
* Select an active instance (in the runtime)
* Go to the "Variables" tab in the bottom
* Click "Add variable", select type string but leave the value field empty. This will create a variable with _type_ "null" and the value column is empty:

* Select the variable line
* Click "Update variable"
* Enter a value
* Click the "Update" button.

The value (and type) doesn't change. Probably because of the type set to null even though it was first selected as "string". It is however possible to change the value through the [update variable REST API](https://www.flowable.com/open-source/docs/bpmn/ch15-REST/#update-a-single-variable-on-a-process-instance).

**Expected behavior**
The value should update. Maybe in case of a current type/value "null", the type select field should also be displayed in the "Update variable" form first as it is in the "Add variable" form.

**Code**
n/a

**Additional context**
Tested in Flowable v6.7.2 and v6.5.0

Riprodurre il problema in flowable-admin tramite Process Engine > Instances > Variables, creando una variabile stringa con un valore vuoto e usando quindi Update variable. Tracciare il form di aggiornamento e confrontarne il comportamento con il form Add variable e con la Update variable REST API; il lavoro è completato quando a una variabile con valore null può essere assegnato un valore e il tipo visualizzato viene aggiornato correttamente.

Scritto dal modello di indicizzazione a partire dal testo della issue.