Issue Summary

A capability can have empty values  while editing via manually in the UI or via bamboo-capabilities.properties.

The API Update existing agent capability  returns a 400 error when empty values set for capability. 

--data '{"key": "test", "value": ""}'  

Steps to Reproduce

Use the REST API to add a capability value .

 $curl -u admin:$pass -X PUT --header 'Content-Type: application/json' <BambooUrl>/rest/api/latest/agent/688129/capability/test --data '{"key": "test", "value": ""}' 

Expected Results

The API should accept empty string values, consistent with:
1. The Bamboo UI, which allows creating/editing capabilities with empty values
2. The bamboo-capabilities.properties file, which accepts test= (empty value)
3. The GET endpoint, which returns capabilities with empty values in the response 

Actual Results

 % curl -X PUT <BAMBOO_URL>/bamboo/rest/api/latest/agent/688129/capability/test  -uadmin  --data '{
  "key": "test",
  "value": ""
}'  --header 'Content-Type: application/json'
Enter host password for user 'admin':
{"errors":[],"fieldErrors":{"capabilityValue":["Need to add a value for the value field"]}}%       

Workaround

Currently there is no known workaround for this behavior via REST API. You can add them via Agent Capabilities screen or use the bamboo-capabilities.properties file.