ContainerRegistry: Add test for empty reference

Author: euanh

Sources/ContainerRegistry/ImageReference.swift

@@ -14,7 +14,7 @@
 
 import RegexBuilder
 
-public enum ReferenceError: Error {
+public enum ReferenceError: Error, Equatable {
     case emptyString
     case unexpected(String)
 }

Tests/ContainerRegistryTests/ImageReferenceTests.swift

@@ -121,14 +121,21 @@ struct ReferenceTests {
     ]
 
     @Test(arguments: tests)
-    func testReferences(test: ReferenceTestCase) throws {
+    func testValidReferences(test: ReferenceTestCase) throws {
         let parsed = try! ImageReference(fromString: test.reference, defaultRegistry: "default")
         #expect(
             parsed == test.expected,
             "\(String(reflecting: parsed)) is not equal to \(String(reflecting: test.expected))"
         )
     }
 
+    @Test
+    func testInvalidReferences() throws {
+        #expect(throws: ReferenceError.emptyString) {
+            try ImageReference(fromString: "", defaultRegistry: "default")
+        }
+    }
+
     @Test
     func testLibraryReferences() throws {
         // docker.io is a special case, as references such as "swift:slim" with no registry component are translated to "docker.io/library/swift:slim"