cgen: fix default zero in map fields (fix #7328) (#7394)

vlib/builtin/map_test.v

@@ -440,3 +440,16 @@ fn test_map_clone() {
 	assert nums2['foo'] == 2
 	assert nums2['bar'] == 8
 }
+
+struct MValue {
+    name string
+    misc map[string]string
+}
+
+fn test_map_default_zero() {
+    m := map[string]MValue{}
+    v := m['unknown']
+    x := v.misc['x']
+    println(x)
+	assert x == ''
+}

vlib/v/gen/cgen.v

@@ -5414,7 +5414,24 @@ fn (mut g Gen) type_default(typ_ table.Type) string {
 	// User struct defined in another module.
 	// if typ.contains('__') {
 	if sym.kind == .struct_ {
-		return '{0}'
+		mut has_array_map := false
+		mut zero_str := '{'
+		info := sym.info as table.Struct
+		for field in info.fields {
+			field_sym := g.table.get_type_symbol(field.typ)
+			if field_sym.kind in [.array, .map] {
+				zero_str += '.$field.name=${g.type_default(field.typ)},'
+				has_array_map = true
+			}
+		}
+		if has_array_map {
+			zero_str += '}'
+			type_name := g.typ(typ)
+			zero_str = '($type_name)' + zero_str
+		} else {
+			zero_str += '0}'
+		}
+		return zero_str
 	}
 	// if typ.ends_with('Fn') { // TODO
 	// return '0'