Fix missing return lint (#9246)

A recent Dart roll improved the detection of missing return statements.
Since we enforce that all enum values must be covered in switch
statements, this isn't an issue, but we add a return to fix the lint.

Author: cbracken

ci/licenses_golden/tool_signature

@@ -1,2 +1,2 @@
-Signature: ad6590b867473860a1e6b08bf9e98667
+Signature: 5b10e36374c7d600de6c632fde955f7d
 

tools/licenses/lib/licenses.dart

@@ -176,6 +176,7 @@ abstract class License implements Comparable<License> {
     if (!reformatted)
       body = _reformat(body);
     final License result = _registry.putIfAbsent(body, () {
+      assert(type != null);
       switch (type) {
         case LicenseType.bsd:
         case LicenseType.mit:
@@ -200,6 +201,7 @@ abstract class License implements Comparable<License> {
         case LicenseType.libpng:
           return BlankLicense._(body, type, origin: origin);
       }
+      return null;
     });
     assert(result.type == type);
     return result;