Avoid division by zero

[amir]

In some scenario the advisor fails with the following stack trace:

Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/Users/amir/Sources/rocksdb/tools/advisor/advisor/rule_parser_example.py", line 98, in <module>
    main(args)
  File "/Users/amir/Sources/rocksdb/tools/advisor/advisor/rule_parser_example.py", line 42, in main
    triggered_rules = rule_spec_parser.get_triggered_rules(
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/amir/Sources/rocksdb/tools/advisor/advisor/rule_parser.py", line 479, in get_triggered_rules
    self.trigger_conditions(data_sources)
  File "/Users/amir/Sources/rocksdb/tools/advisor/advisor/rule_parser.py", line 496, in trigger_conditions
    source.check_and_trigger_conditions(cond_subset)
  File "/Users/amir/Sources/rocksdb/tools/advisor/advisor/db_timeseries_parser.py", line 148, in check_and_trigger_conditions
    result = self.fetch_burst_epochs(
             ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/amir/Sources/rocksdb/tools/advisor/advisor/db_timeseries_parser.py", line 88, in fetch_burst_epochs
    diff = diff * 100 / first_val
           ~~~~~~~~~~~^~~~~~~~~~~
ZeroDivisionError: float division by zero

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