Don't use version to check for a not-found fallback

In _find_cached_fallback_dep(), actually check if dependency is not found,
rather than using a specific version as a proxy for that.

mesonbuild/interpreter.py

@@ -2822,7 +2822,7 @@ external dependencies (including libraries) must go to "dependencies".''')
             return False
         found = dep.version_method([], {})
         # Don't do a version check if the dependency is not found and not required
-        if found == 'none' and not required:
+        if not dep.found_method([], {}) and not required:
             subproj_path = os.path.join(self.subproject_dir, dirname)
             mlog.log('Dependency', mlog.bold(name), 'from subproject',
                      mlog.bold(subproj_path), 'found:', mlog.red('NO'), '(cached)')