Handle case where Python main script is undefined

In the Python runtime's launch function, we don't fail fast when the
mainProgram is null, because there are cases where you might want that,
such as running `python -i` to bring up the REPL. So let's not print
"null" for the main program name in that case, but rather omit it.

src/commonMain/kotlin/python.kt

@@ -118,7 +118,7 @@ class PythonRuntimeConfig(recognizedArgs: Array<String>) :
         dryRun(buildString {
             append(python?.binPython ?: "python")
             args.runtime.forEach { append(" $it") }
-            append(" $mainProgram")
+            if (mainProgram != null) append(" $mainProgram")
             args.main.forEach { append(" $it") }
         })