encoding/jsonschema: add test case for empty field

I noticed that empty fields were inadvertently missed from some property
checks due to a flaw in the `patternProperties` logic. Add a test case
for this case, to be fixed in a subsequent CL.

Signed-off-by: Roger Peppe <[email protected]>
Change-Id: I32e8341edb3ee73cdcbfa406ff4d51b2ce634c66
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1203596
Unity-Result: CUE porcuepine <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>

encoding/jsonschema/testdata/txtar/pattern_with_empty_field.txtar

@@ -0,0 +1,21 @@
+# This tests the case where there's a patternProperties keyword
+# and no other fields.
+# TODO the empty-field test should fail but currently does not.
+
+-- schema.json --
+{
+    "$schema": "https://json-schema.org/draft/2020-12/schema",
+    "type": "object",
+    "patternProperties": {
+    	".*": {
+    		"type": "string"
+    	}
+    }
+}
+-- out/decode/extract --
+@jsonschema(schema="https://json-schema.org/draft/2020-12/schema")
+
+{[=~".*" & !~"^()$"]: string}
+...
+-- test/empty-field.json --
+{"": true}