Dont try to parse content if it is None, as throws an unnecessary exc…

…eption (this can happend for a lot of reasons, in my case it was an SSL fail)

locust/contrib/fasthttp.py

@@ -254,7 +254,10 @@ def text(self):
             # A TypeError can be raised if encoding is None
             #
             # Fallback to decode without specifying encoding
-            content = unicode(self.content, errors='replace')
+            if self.content is None:
+                content = None
+            else:
+                content = unicode(self.content, errors='replace')
         return content
 
     @property