Fix signing settings when no Keychain is specified (#1093)

Closes #1086

gradle-plugins/compose/src/main/kotlin/org/jetbrains/compose/desktop/application/internal/validation/ValidatedMacOSSigningSettings.kt

@@ -44,10 +44,10 @@ internal fun MacOSSigningSettings.validate(
     val signIdentity = this.identity.orNull
         ?: error(ERR_UNKNOWN_SIGN_ID)
     val keychainPath = this.keychain.orNull
-    val keychainFile =
-        listOf(project.file(keychainPath), project.rootProject.file(keychainPath))
-            .firstOrNull { it.exists() }
     if (keychainPath != null) {
+        val keychainFile =
+            listOf(project.file(keychainPath), project.rootProject.file(keychainPath))
+                .firstOrNull { it.exists() }
         check(keychainFile != null && keychainFile.exists()) {
             "$ERR_PREFIX could not find the specified keychain: $keychainPath"
         }