JVM IR: Support parameterless main methods

This commit:

- introduces tests explicating what is and isn't considered a
  proper main method on the JVM backends.
- implements support for parameterless main methods on the JVM IR
  backend
- See KT-34338 for more tests.

compiler/testData/codegen/bytecodeText/parameterlessMain/dontGenerateOnNullableArray.kt

@@ -0,0 +1,7 @@
+fun main() {
+
+}
+
+fun main(args: Array<String>?) {
+
+}