Fix assert(not true) raising backtrace

When no message is provided to assert(), it uses the ast printer to show
the condition that failed. In this case the 'not' is the first string
appended to the result, self.result[-1] would raise range error.

mesonbuild/ast/printer.py

@@ -48,7 +48,7 @@ class AstPrinter(AstVisitor):
         self.is_newline = False
 
     def append_padded(self, data: str, node: mparser.BaseNode) -> None:
-        if self.result[-1] not in [' ', '\n']:
+        if self.result and self.result[-1] not in [' ', '\n']:
             data = ' ' + data
         self.append(data + ' ', node)