Macro-processor: more robust handling of empty files

The support for empty files at the macro-processor level, as implemented in
12913200, was relying on
basic_streambuf::in_avail(), which does not seem to behave consistently across
platforms, and which may not be the right tool for the job.

Rather use the Bison grammar to support empty files.

Closes: #93

src/macro/Driver.cc

 /*
- * Copyright © 2019-2020 Dynare Team
+ * Copyright © 2019-2022 Dynare Team
  *
  * This file is part of Dynare.
  *
@@ -40,10 +40,6 @@ Driver::parse(const string &file_arg, const string &basename_arg, const istream
       m.parse("command_line_defines", "command_line_defines", is, debug, {}, env, paths, output);
     }
 
-  // Handle empty files
-  if (modfile.rdbuf()->in_avail() == 0)
-    return;
-
   stringstream file_with_endl;
   file_with_endl << modfile.rdbuf() << endl;
 

src/macro/Parser.yy

 // -*- C++ -*-
 /*
- * Copyright © 2019-2021 Dynare Team
+ * Copyright © 2019-2022 Dynare Team
  *
  * This file is part of Dynare.
  *
@@ -105,7 +105,11 @@ using namespace macro;
 
 %%
 
-%start statements;
+%start statements_or_empty_file;
+
+statements_or_empty_file : %empty
+                         | statements
+                         ;
 
 statements : statement
              {