[FIR] Add createEmptySession to FirSessionFactory for testing purposes

compiler/fir/entrypoint/src/org/jetbrains/kotlin/fir/session/FirSessionFactory.kt

@@ -9,7 +9,9 @@ import com.intellij.openapi.extensions.Extensions
 import com.intellij.openapi.project.Project
 import com.intellij.psi.PsiElementFinder
 import com.intellij.psi.search.GlobalSearchScope
+import org.jetbrains.annotations.TestOnly
 import org.jetbrains.kotlin.analyzer.ModuleInfo
+import org.jetbrains.kotlin.fir.FirSession
 import org.jetbrains.kotlin.fir.PrivateSessionConstructor
 import org.jetbrains.kotlin.fir.java.*
 import org.jetbrains.kotlin.fir.java.deserialization.KotlinDeserializedJvmSymbolsProvider
@@ -101,4 +103,9 @@ object FirSessionFactory {
             )
         }
     }
+
+    @TestOnly
+    fun createEmptySession(): FirSession {
+        return object : FirSession(null) {}
+    }
 }