Check for empty expression properly

The old code was a left-over from the times when the
`capturedExpression` member was a `const char*`, which could always
be indexed. With the change to use `StringRef`, blindly indexing 0th
element is invalid, as it is not indexable part of a StringRef.

include/internal/catch_assertionresult.cpp

@@ -45,7 +45,7 @@ namespace Catch {
     }
 
     bool AssertionResult::hasExpression() const {
-        return m_info.capturedExpression[0] != 0;
+        return !m_info.capturedExpression.empty();
     }
 
     bool AssertionResult::hasMessage() const {