Follow-up from "Fix bug with resolving values and feature flag"

Currently, when pipeline inputs are submitted via the web form with empty values, the frontend sends value: "" (empty string) rather than null or omitting the field. We handle this in the input validation layer Ci::Inputs::BaseInput by converting empty strings to nil for rules-based inputs:

param = nil if param.is_a?(String) && param.empty? && rules

this leaks form submission details into the validation layer.

The following discussion from !213358 (merged) should be addressed:

• @avielle started a discussion: (+4 comments)

  From what I can tell, we're still changing existing behavior here. Currently this is valid:

  spec:
    inputs:
      test_input:
        default: ""

  With this change default: "" will no longer be valid. Is there a reason that we need empty string values to be invalid when using rules?