[clang-tidy] Use .empty instead of comparing size

Found with readability-container-size-empty

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/Eval.cpp

@@ -218,7 +218,7 @@ void Eval::evaluatePostfixStack (
   const std::vector <std::pair <std::string, Lexer::Type>>& tokens,
   Variant& result) const
 {
-  if (tokens.size () == 0)
+  if (tokens.empty())
     throw std::string ("No expression to evaluate.");
 
   // This is stack used by the postfix evaluator.
@@ -229,7 +229,7 @@ void Eval::evaluatePostfixStack (
     if (token.second == Lexer::Type::op &&
         token.first == "!")
     {
-      if (values.size () < 1)
+      if (values.empty())
         throw std::string ("The expression could not be evaluated.");
 
       Variant right = values.back ();
@@ -242,7 +242,7 @@ void Eval::evaluatePostfixStack (
     else if (token.second == Lexer::Type::op &&
              token.first == "_neg_")
     {
-      if (values.size () < 1)
+      if (values.empty())
         throw std::string ("The expression could not be evaluated.");
 
       Variant right = values.back ();
@@ -755,14 +755,14 @@ void Eval::infixToPostfix (
     else if (token.second == Lexer::Type::op &&
              token.first == ")")
     {
-      while (op_stack.size () &&
+      while (!op_stack.empty() &&
              op_stack.back ().first != "(")
       {
         postfix.push_back (op_stack.back ());
         op_stack.pop_back ();
       }
 
-      if (op_stack.size ())
+      if (!op_stack.empty())
         op_stack.pop_back ();
       else
         throw std::string ("Mismatched parentheses in expression");
@@ -773,7 +773,7 @@ void Eval::infixToPostfix (
       char type2;
       unsigned int precedence2;
       char associativity2;
-      while (op_stack.size () > 0 &&
+      while (!op_stack.empty() &&
              identifyOperator (op_stack.back ().first, type2, precedence2, associativity2) &&
              ((associativity == 'l' && precedence <= precedence2) ||
               (associativity == 'r' && precedence <  precedence2)))
@@ -790,7 +790,7 @@ void Eval::infixToPostfix (
     }
   }
 
-  while (op_stack.size ())
+  while (!op_stack.empty())
   {
     if (op_stack.back ().first == "(" ||
         op_stack.back ().first == ")")