DrupalWebTestCase::assertFieldByName() outputs bogus assertion message when no $value is passed

Problem/Motivation

• DrupalWebTestCase::assertFieldByName() produces confusing error text when the $value parameter has the default value of NULL, e.g.:
  Found field with name logo_path and value "

Proposed resolution

• Display a different message when the value is not set.

Original report by sun

Extracted from #1376166: Custom logo and favicon functionality inanely tries to support absolute local file paths