make optional fields possible for input framework.

This do not have to be present in the input file and are marked as &optional in the record description.

Those can e.g. be used to create field values on the file in a predicate while reading a file - example:

	Input::add_table([$source="input.log", $name="input", $idx=Idx, $val=Val, $destination=servers,
				$pred(typ: Input::Event, left: Idx, right: Val) = { right$notb = !right$b; return T; }

testing/btest/scripts/base/frameworks/input/optional.bro

@@ -0,0 +1,45 @@
+#
+# @TEST-EXEC: bro -b %INPUT >out
+# @TEST-EXEC: btest-diff out
+
+@TEST-START-FILE input.log
+#separator \x09
+#path	ssh
+#fields	i	b	
+#types	int	bool
+1	T
+2	T
+3	F
+4	F
+5	F
+6	F
+7	T
+@TEST-END-FILE
+
+redef InputAscii::empty_field = "EMPTY";
+
+module A;
+
+type Idx: record {
+	i: int;
+};
+
+type Val: record {
+	b: bool;
+	notb: bool &optional;
+};
+
+global servers: table[int] of Val = table();
+
+event bro_init()
+{
+	# first read in the old stuff into the table...
+	Input::add_table([$source="input.log", $name="input", $idx=Idx, $val=Val, $destination=servers, 
+				$pred(typ: Input::Event, left: Idx, right: Val) = { right$notb = !right$b; return T; }
+				]);
+	Input::remove("input");
+}
+
+event Input::update_finished(name: string, source: string) {
+	print servers;
+}