perf scripts python: export-to-postgresql.py: Fix invalid input syntax for integer error

Fix SQL query error "invalid input syntax for integer":

  Traceback (most recent call last):
    File "tools/perf/scripts/python/export-to-postgresql.py", line 465, in <module>
      do_query(query, 'CREATE VIEW calls_view AS '
    File "tools/perf/scripts/python/export-to-postgresql.py", line 274, in do_query
      raise Exception("Query failed: " + q.lastError().text())
  Exception: Query failed: ERROR:  invalid input syntax for integer: ""
  LINE 1: ...ch_count,call_id,return_id,CASE WHEN flags=0 THEN '' WHEN fl...
                                                               ^
  (22P02) QPSQL: Unable to create query
  Error running python script tools/perf/scripts/python/export-to-postgresql.py

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Fixes: f08046cb ("perf thread-stack: Represent jmps to the start of a different symbol")
Link: https://lkml.kernel.org/n/tip-strfpdozrvg7bi1xzrivxzqt@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>