Calling evaluate with empty (or no) parameter will log a "Expression is emtpy." message

This message is retrieved via `te_parser::get_last_error_message()`.
Closes #19

Author: Blake-Madden

tests/tetests.cpp

@@ -1086,6 +1086,25 @@ TEST_CASE("Copy", "[copy]")
         }
     }
 
+TEST_CASE("Empty", "[empty]")
+    {
+    te_parser tep;
+
+    SECTION("Empty Eval")
+        {
+        CHECK(std::isnan(tep.evaluate()));
+        CHECK_FALSE(tep.success());
+        CHECK(std::string{ "Expression is emtpy." } == tep.get_last_error_message());
+        }
+
+    SECTION("Empty String")
+        {
+        CHECK(std::isnan(tep.evaluate("")));
+        CHECK_FALSE(tep.success());
+        CHECK(std::string{ "Expression is emtpy." } == tep.get_last_error_message());
+        }
+    }
+
 TEST_CASE("Inf", "[inf]")
     {
     te_parser tep;

tinyexpr.cpp

@@ -2484,7 +2484,7 @@ bool te_parser::compile(const std::string_view expression)
     reset_state();
     if (get_list_separator() == get_decimal_separator())
         {
-        throw std::runtime_error("List and decimal separators cannot be the same");
+        throw std::runtime_error("List and decimal separators cannot be the same.");
         }
     if (expression.empty())
         {
@@ -2561,6 +2561,12 @@ te_type te_parser::evaluate()
     {
     try
         {
+        if (m_expression.empty())
+            {
+            m_parseSuccess = false;
+            m_errorPos = 0;
+            m_lastErrorMessage = "Expression is emtpy.";
+            }
         m_result = (m_compiledExpression != nullptr) ? te_eval(m_compiledExpression) : te_nan;
         }
     catch (const std::exception& expt)
@@ -2583,6 +2589,14 @@ te_parser::evaluate(const std::string_view expression) // NOLINT(-readability-id
         {
         return evaluate();
         }
+
+    if (expression.empty())
+        {
+        m_parseSuccess = false;
+        m_errorPos = 0;
+        m_lastErrorMessage = "Expression is emtpy.";
+        }
+
     return te_nan;
     }