input/Manager: Ignore empty record types

Somewhere record types with zero fields get the optional attribute
apparently. The input/sqlite/basic test failed due to complaining
that ctx is optional. It isn't optional and when it has zero fields
we can just ignore it, too.

Also adds a input framework test with an explicit empty record type

testing/btest/scripts/base/frameworks/input/empty-nested-record.zeek

@@ -0,0 +1,50 @@
+# @TEST-DOC: Check that empty record types are ignored.
+#
+# @TEST-EXEC: zeek -b %INPUT >out
+# @TEST-EXEC: btest-diff .stderr
+# @TEST-EXEC: btest-diff out
+
+# @TEST-START-FILE input.log
+#separator \x09
+#path	ssh
+#fields	s	r0.c0	r0.c1	r0.s
+#types	string	count	string
+string1	4242	4711	r0s-1
+string2	4343	-	r0s-2
+# @TEST-END-FILE
+
+redef exit_only_after_terminate = T;
+
+module Test;
+
+type EmptyRec: record { };
+
+type MyRec: record {
+	e0: EmptyRec;  # stuffing
+	c0: count;
+	c1: count &optional;
+	e1: EmptyRec;  # stuffing
+	s: string;
+};
+
+type Val: record {
+	s: string;
+	e0: EmptyRec;  # stuffing
+	r0: MyRec;
+	e1: EmptyRec;  # stuffing
+};
+
+event Test::line(description: Input::EventDescription, tpe: Input::Event, v: Val)
+	{
+	print tpe, v;
+	}
+
+event Input::end_of_data(name: string, source:string)
+	{
+	terminate();
+	}
+
+event zeek_init()
+	{
+	Input::add_event([$source="input.log", $name="file", $fields=Val, $ev=Test::line]);
+	}