]configure # setting without colon sets the setting to '' #765

Describe the bug
]configure # setting without colon sets the setting to ''

To Reproduce
Steps to reproduce the behaviour:

1. ]create # \tmp\colon
2. ]configure # text:plain
3. ]configure # text
4. ]configure # text
5. Notice that the value was changed to nothing

Expected behaviour
Only report, don't change

Desktop (please complete the following information):

• OS: Win
• APL version: 20.0.53342
• ⎕SE.Link.Version: 4.1.6