With accent marks:
Свя́то ме́сто пу́сто не быва́ет.
A holy place is never empty.
If a good place is empty, someone else immediately takes it.
For example, a position promising benefits and tempting prospects is rarely vacant - there are always those who want to take it.
Other Russian proverbs
Fear has big eyes.
~ Seven babysitters make the kid to lose an eye.
Eggs don't teach the chicken.
Live for a century, learn for a century, but die being a fool anyway.
Don't bring your set of rules in a foreign monastery
Forewarned is forearmed.
Get your sleigh ready in summer and your (horse-drawn) cart in winter.
One head is good, but two ones are better.