Fix empty file parsing

Pointer::start was modified to store line without trailing whitespaces.
So we need extra step to check if the file isn't empty during token
recognization.

Author: weirdsmiley

src/lexer.rs

@@ -244,6 +244,11 @@ impl Lexer {
     /// character at end, so we must keep calling `next_line` until a non-empty
     /// `self.line` is returned.
     pub(crate) fn next_token(&mut self) -> Result<Option<Token>> {
+        if self.buffer.is_empty() {
+            self.token = None;
+            return Ok(self.token);
+        }
+
         // Skip all leading whitespaces and trailing newlines.
         while self.buffer[self.ptr.current].is_ascii_whitespace() {
             self.ptr.current += 1;

tests/test_empty.ql

@@ -292,6 +292,10 @@ fn test_ast_gen() -> Result<(), Box<dyn std::error::Error>>  {
     |_ mix5: float64 = (1 >= w: float64)
     |_ (a0: float64 + (a1: float64 + (a2: float64 + a3: float64)))
 
+");
+
+    test!("tests/test_empty.ql",
+"|_ test_empty			// @test_empty.ql:1:1
 ");
 
     test!("examples/toss.ql",