A sol file can be empty

Signed-off-by: Sean Young <sean@mess.org>

src/parser/solidity.lalrpop

@@ -12,7 +12,7 @@ use super::doc::tags;
 grammar<'input>(input: &'input str, file_no: usize);
 
 pub SourceUnit: SourceUnit = {
-    SourceUnitPart+ => SourceUnit(<>)
+    SourceUnitPart* => SourceUnit(<>)
 }
 
 SourceUnitPart: SourceUnitPart = {

tests/substrate_tests/first.rs

@@ -1,10 +1,16 @@
 use parity_scale_codec::Encode;
 use parity_scale_codec_derive::{Decode, Encode};
 
-use crate::build_solidity;
+use crate::{build_solidity, no_errors, parse_and_resolve};
+use solang::Target;
 
 #[test]
 fn simple_solidiy_compile_and_run() {
+    // try empty file
+    let ns = parse_and_resolve("", Target::Substrate);
+
+    no_errors(ns.diagnostics);
+
     #[derive(Debug, PartialEq, Encode, Decode)]
     struct FooReturn {
         value: u32,