cgen: skip deleted map entries in the auto generated .str() method

vlib/builtin/map_test.v

@@ -339,3 +339,17 @@ fn test_mut_map_with_relation_op_in_fn() {
 	assert 'seven' in m
 	assert 'eight' in m
 }
+
+fn test_map_str_after_delete() {
+	mut m := {
+		'first': 1,
+		'second': 2,
+		'third': 3,
+	}
+	osm := '$m'
+	m.delete('second')
+	nsm := '$m'
+    println('m: $m')
+	assert osm == "{'first': 1, 'second': 2, 'third': 3}"
+	assert nsm == "{'first': 1, 'third': 3}"
+}

vlib/v/gen/auto_str_methods.v

@@ -256,6 +256,7 @@ fn (mut g Gen) gen_str_for_map(info table.Map, styp string, str_fn_name string)
 	g.auto_str_funcs.writeln('\tstrings__Builder sb = strings__new_builder(m.key_values.len*10);')
 	g.auto_str_funcs.writeln('\tstrings__Builder_write(&sb, tos_lit("{"));')
 	g.auto_str_funcs.writeln('\tfor (unsigned int i = 0; i < m.key_values.len; ++i) {')
+	g.auto_str_funcs.writeln('\t\tif (m.key_values.keys[i].str == 0) { continue; }')
 	g.auto_str_funcs.writeln('\t\tstring key = (*(string*)DenseArray_get(m.key_values, i));')
 	g.auto_str_funcs.writeln('\t\tstrings__Builder_write(&sb, _STR("\'%.*s\\000\'", 2, key));')
 	g.auto_str_funcs.writeln('\t\tstrings__Builder_write(&sb, tos_lit(": "));')