LU-10834 tests: fix cleanup_77c() defect

LU-10834 tests: fix cleanup_77c() defect

Check the length of string is non-zero properly.
Without this fix cleanup_77c() calls:
 "rm -f *" in case of empty $osc_file_prefix, and
 "do_facet ost1 rm -f *" in case of empty $ost_file_prefix

Signed-off-by: Elena Gryaznova <c17455@cray.com>
Cray-bug-id: LUS-5817
Test-Parameters: trivial testlist=sanity envdefinitions="ONLY=77c"
Reviewed-by: Vladimir Saveliev <c17830@cray.com>
Reviewed-by: Andrew Perepechko <c17827@cray.com>
Change-Id: Id3f3ec52c965b15900f7a649120b89267daa9810
Reviewed-on: https://review.whamcloud.com/31918
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>