parser: fix empty import error

vlib/v/parser/parser.v

@@ -302,6 +302,9 @@ pub fn (mut p Parser) top_stmt() ast.Stmt {
 		}
 		.key_import {
 			node := p.import_stmt()
+			if node.len == 0 {
+				return p.top_stmt()
+			}
 			p.ast_imports << node
 			return node[0]
 		}

vlib/v/tests/inout/empty_import.out

@@ -0,0 +1 @@
+empty import

vlib/v/tests/inout/empty_import.vv

@@ -0,0 +1,7 @@
+import (
+	// no module
+)
+
+fn main() {
+	println('empty import')
+}