machine file: Fix crash on empty value

Fixes: #12025

mesonbuild/coredata.py

@@ -992,9 +992,11 @@ class MachineFileParser():
             value = value.replace('\\', '\\\\')
             try:
                 ast = mparser.Parser(value, 'machinefile').parse()
+                if not ast.lines:
+                    raise EnvironmentException('value cannot be empty')
                 res = self._evaluate_statement(ast.lines[0])
-            except MesonException:
-                raise EnvironmentException(f'Malformed value in machine file variable {entry!r}.')
+            except MesonException as e:
+                raise EnvironmentException(f'Malformed value in machine file variable {entry!r}: {str(e)}.')
             except KeyError as e:
                 raise EnvironmentException(f'Undefined constant {e.args[0]!r} in machine file variable {entry!r}.')
             section[entry] = res