Improve license detection

src/licensedcode/cache.py

@@ -515,6 +515,9 @@ def build_spdx_license_expression(license_expression, licensing=None):
     >>> spdx = "MIT OR GPL-2.0-only WITH LicenseRef-scancode-generic-exception"
     >>> assert build_spdx_license_expression(exp) == spdx
     """
+    if not license_expression:
+        return
+
     if not licensing:
         licensing = get_licensing()
     validate_spdx_license_keys(license_expression=license_expression, licensing=licensing)

src/licensedcode/data/rules/lead-in_unknown_43.RULE

@@ -1,6 +1,6 @@
 ---
 license_expression: unknown-license-reference
-is_license_tag: yes
+is_license_clue: yes
 relevance: 60
 notes: Creative commons tag seen in RDF or XML documents
 ---