Do not output empty non-root packages in tests with txt

Mostly to avoid listing every internal package in
kotlin-stdlib/kotlin-reflect, as in
https://github.com/JetBrains/kotlin/blob/acd5b62148d797ffd24c4811d1d663f0c1ae7885/compiler/testData/diagnostics/testsWithUnsignedTypes/conversions/conversionOfSignedToUnsigned.txt#L144

compiler/testData/diagnostics/tests/sourceCompatibility/apiVersion/typealiasesOnImport.txt

@@ -11,6 +11,3 @@ package a {
     @kotlin.SinceKotlin(version = "1.1") public typealias Alias_1_1 = kotlin.String
     public typealias Since_1_1_Alias = a.Since_1_1
 }
-
-package b {
-}