Validate: Add empty() and coerce() methods

Implementing the undocumented coerce_for_json() method from TUWF and
elm_empty() from VNDB.

t/validate-util.t

@@ -0,0 +1,36 @@
+use v5.36;
+use Test::More;
+use FU::Validate;
+use FU::Util 'json_format';
+
+my $schema = FU::Validate->compile({ keys => {
+    bool => { anybool => 1 },
+    num => { num => 1 },
+    int => { int => 1 },
+    str => { default => 'x' },
+    intarray => { elems => { int => 1 } },
+    any => { type => 'any' },
+}});
+
+
+is json_format($schema->coerce(undef)), 'null';
+is json_format($schema->coerce("str")), '"str"';
+
+is json_format($schema->coerce({
+        bool => 'abc',
+        num => " 1.5 ",
+        int => 9.7,
+        str => !1,
+        intarray => [ 1.5, -10, undef, ' 0E0 ' ],
+        any => {},
+        whatsthis => undef,
+    }, unknown => 'remove'), canonical => 1),
+    '{"any":{},"bool":true,"int":9,"intarray":[1,-10,null,0],"num":1.5,"str":""}';
+
+is json_format($schema->coerce({uhm => 1}), canonical => 1),
+    '{"any":null,"bool":false,"int":0,"intarray":[],"num":0,"str":"x","uhm":1}';
+
+is json_format($schema->empty, canonical => 1),
+    '{"any":null,"bool":false,"int":0,"intarray":[],"num":0,"str":"x"}';
+
+done_testing;