collectors/mdstat: return empty dict if MDSTAT_PATH can't be opened for reading
collectors/mdstat: return empty dict if MDSTAT_PATH can't be opened for reading

Signed-off-by: julian <34c6fceca75e456f25e7e99531e2425c6c1de443@jneureuther.de>

skbkontur/Diamond,python-diamond/Diamond,python-diamond/Diamond,gg7/diamond,tusharmakkar08/Diamond,gg7/diamond,gg7/diamond,python-diamond/Diamond,MichaelDoyle/Diamond,skbkontur/Diamond,tusharmakkar08/Diamond,skbkontur/Diamond,MichaelDoyle/Diamond,skbkontur/Diamond,tusharmakkar08/Diamond,MichaelDoyle/Diamond,MichaelDoyle/Diamond,tusharmakkar08/Diamond,gg7/diamond