Add empty string check to LUKS2 JSON validation.

Most of the LUKS2 fields cannot be empty,
add check for JSON validation for it to fail early.

Fixes: #746

tests/generators/generate-luks2-segment-crypt-empty-encryption.img.sh

@@ -0,0 +1,39 @@
+#!/bin/bash
+
+. lib.sh
+
+#
+# *** Description ***
+#
+# generate primary header with segment empty encryption field
+#
+# secondary header is corrupted on purpose as well
+#
+
+# $1 full target dir
+# $2 full source luks2 image
+
+function generate()
+{
+	# remove mandatory encryption field
+	json_str=$(jq -c '.segments."0".encryption = ""' $TMPDIR/json0)
+	test ${#json_str} -lt $((LUKS2_JSON_SIZE*512)) || exit 2
+
+	write_luks2_json "$json_str" $TMPDIR/json0
+
+	lib_mangle_json_hdr0_kill_hdr1
+}
+
+function check()
+{
+	lib_hdr1_killed || exit 2
+
+	read_luks2_json0 $TGT_IMG $TMPDIR/json_res0
+	jq -c 'if .segments."0".encryption != ""
+	       then error("Unexpected value in result json") else empty end' $TMPDIR/json_res0 || exit 5
+}
+
+lib_prepare $@
+generate
+check
+lib_cleanup