Print whole history on `%pycat` without parameters

The only thing required here was removal of the empty argument guard, as
`InteractiveShell.find_user_code` already understands empty string as
"complete history of current session without the last line".

IPython/core/magics/osm.py

@@ -808,17 +808,17 b' class OSMagics(Magics):'
         This magic command can either take a local filename, an url,
         an history range (see %history) or a macro as argument ::
 
+        If no parameter is given, prints out history of current session up to
+        this point.
+
         %pycat myscript.py
         %pycat 7-27
         %pycat myMacro
         %pycat http://www.example.com/myscript.py
         """
-        if not parameter_s:
-            raise UsageError('Missing filename, URL, input history range, '
-                             'or macro.')
-
-        try :
-            cont = self.shell.find_user_code(parameter_s, skip_encoding_cookie=False)
+        try:
+            cont = self.shell.find_user_code(parameter_s,
+                                             skip_encoding_cookie=False)
         except (ValueError, IOError):
             print("Error: no such file, variable, URL, history range or macro")
             return