avoid index access in KtFile.getClasses() when the file isn't part of a multi-file class (helps with KT-10684)

idea/testData/asJava/fileLightClass/multifile1.kt

@@ -0,0 +1,6 @@
+@file:kotlin.jvm.JvmMultifileClass
+@file:kotlin.jvm.JvmName("Foo")
+
+fun foo() {
+}
+

idea/testData/asJava/fileLightClass/multifile2.kt

@@ -0,0 +1,6 @@
+@file:kotlin.jvm.JvmMultifileClass
+@file:kotlin.jvm.JvmName("Foo")
+
+fun bar() {
+}
+