fix: Address compile warnings on custom Truth subjects (#275)

These were introduced when the Truth version was upgraded in #268.

src/main/kotlin/org/wfanet/measurement/common/crypto/testing/SignatureSubject.kt

@@ -26,17 +26,21 @@ import org.wfanet.measurement.common.crypto.SignatureAlgorithm
 import org.wfanet.measurement.common.crypto.subjectKeyIdentifier
 import org.wfanet.measurement.common.crypto.verifySignature
 
-class SignatureSubject private constructor(failureMetadata: FailureMetadata, subject: ByteString) :
-  Subject(failureMetadata, subject) {
-
-  private val actual = subject
+class SignatureSubject
+private constructor(failureMetadata: FailureMetadata, private val actual: ByteString?) :
+  Subject(failureMetadata, actual) {
 
   fun isValidFor(certificate: X509Certificate, algorithm: SignatureAlgorithm, data: ByteString) {
+    if (actual == null) {
+      failWithActual(Fact.simpleFact("expected not to be null"))
+      return
+    }
+
     if (!certificate.verifySignature(algorithm, data, actual)) {
       failWithActual(
         Fact.simpleFact("cannot be verified"),
         Fact.fact("certificate SKID", certificate.subjectKeyIdentifier),
-        Fact.fact("data", data)
+        Fact.fact("data", data),
       )
     }
   }
@@ -45,7 +49,9 @@ class SignatureSubject private constructor(failureMetadata: FailureMetadata, sub
     fun assertThat(actual: ByteString): SignatureSubject =
       Truth.assertAbout(signatures()).that(actual)
 
-    fun signatures(): (failureMetadata: FailureMetadata, subject: ByteString) -> SignatureSubject =
-      ::SignatureSubject
+    fun signatures() =
+      Factory<SignatureSubject, ByteString> { metadata, actual ->
+        SignatureSubject(metadata, actual)
+      }
   }
 }

src/main/kotlin/org/wfanet/measurement/common/testing/CommandLineTesting.kt

@@ -67,25 +67,25 @@ object CommandLineTesting {
     assertAbout(CapturedOutputSubject.capturedOutputs()).that(actual)
 
   class CapturedOutputSubject
-  private constructor(
-    metadata: FailureMetadata,
-    private val actual: CapturedOutput,
-  ) : Subject(metadata, actual) {
+  private constructor(metadata: FailureMetadata, private val actual: CapturedOutput?) :
+    Subject(metadata, actual) {
     fun status(): IntegerSubject {
-      return check("status").that(actual.status)
+      return check("status").that(actual?.status)
     }
 
     fun out(): StringSubject {
-      return check("out").that(actual.out)
+      return check("out").that(actual?.out)
     }
 
     fun err(): StringSubject {
-      return check("err").that(actual.err)
+      return check("err").that(actual?.err)
     }
 
     companion object {
-      fun capturedOutputs(): (FailureMetadata, CapturedOutput) -> CapturedOutputSubject =
-        ::CapturedOutputSubject
+      fun capturedOutputs() =
+        Factory<CapturedOutputSubject, CapturedOutput> { metadata, actual ->
+          CapturedOutputSubject(metadata, actual)
+        }
     }
   }
 }
@@ -103,6 +103,7 @@ object ExitInterceptingSecurityManager : SecurityManager() {
   override fun checkPermission(perm: Permission?) {
     // Allow everything.
   }
+
   override fun checkExit(status: Int) {
     super.checkExit(status)