create_perf_json: Support EventCode FIXED

This commit unsets EventCode for FIXED events. Previously for fixed
events the EventCode was 0x00. The get() helper used to process JSON
attributes drops 0x00 (returns None). Here we add an extra check to
also set event_code to None if EventCode is FIXED.

Example FIXED event:
    {
      "EventCode": "FIXED",
      "UMask": "0x01",
      "EventName": "INST_RETIRED.ANY",
      "Counter": "Fixed counter 0",
      "CounterHTOff": "Fixed counter 0",
      <snip>
      "Offcore": "0",
      "Deprecated": "0"
    }

scripts/create_perf_json.py

@@ -353,6 +353,14 @@ def get(key: str) -> str:
             self.event_code = "0xff"
             self.umask = None
 
+        # Unset event_code for fixed core counters. Example:
+        #   "EventCode": "FIXED",
+        #   "EventName": "INST_RETIRED.ANY",
+        #   "Counter": "Fixed counter 0",
+        if "Counter" in jd and "fixed counter" in jd["Counter"].lower():
+            if str(self.event_code).lower() == 'fixed':
+                self.event_code = None
+
         if self.filter:
             remove_filter_start = [
                 "cbofilter",