Declared license field self-changing to NOASSERTION when changing a file-level license

**Issue Type:** Bug
**Issue Description:**

I was attempting to update a file's discovered license on a definition using the SPDX expression editor. But when I did that, it also changed the definition's "_Declared License_" field to "NOASSERTION".

The _Declared License_ on this definition was set by me through a previous curation: https://github.com/clearlydefined/curated-data/pull/2063/commits/b0f91668769d2e0a01598a706b5ffb816f152cfd. My intention was not to change this field again.

**Steps to Reproduce:**

1. Go to the following definition: https://clearlydefined.io/definitions/git/github/jruby/jruby/8a269e34ff3d242ec90fb08061f79c2757a5e157

2. Filter by the following license expression: "_EPL-1.0 OR (GPL-2.0-only OR LGPL-2.1-only)_"

3. On the file _JRubyPOSIXHandler.java_, change the existing license from "EPL-1.0" to "EPL-2.0" (as per actual license header, but leave other existing parts of the file's license expression in tact).

4. Wait a few seconds and you should see the proposed change to the file displayed in the UI (highlighted in blue), but the definition's _Declared License_ field is now changed to "NOASSERTION".


**Further Detail**
Operating System: Windows
Browser: Chrome 76.0 (64-bit)