Utils: Add isEmpty to string view

Change-Id: Ib7e77602dcdcdf481f6ab690bacef53c5dd92580
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/libs/utils/smallstringview.h

@@ -85,6 +85,12 @@ public:
         return m_size;
     }
 
+    constexpr
+    size_type isEmpty() const
+    {
+        return m_size == 0;
+    }
+
     const_iterator begin() const noexcept
     {
         return data();