piwik.js: setCustomVariable should not throw error when value is undefined

[mattab]

When setCustomVariable is called with an undefined value:

Uncaught TypeError: Cannot call method 'slice' of undefined piwik.js:2483
Tracker.setCustomVariable piwik.js:2483
apply piwik.js:545
(anonymous function) piwik.js:3082
(anonymous function)

Spec:

• when custom variable name is undefined, fail silently
• when custom variable value is undefined, change to empty string

[mattab]

In 20a1ff4: Refs #4938 Adding test case which shows the error
this should fail the JavascriptTests travis build

[mattab]

In 6f9b869: Fixes #4938 make sure we deal with undefined parameters
Should make the JavascriptTests build green again