Fix contest grade calculation in contest view: Division by zero could occur...

Fix contest grade calculation in contest view: Division by zero could occur for empty contests after they have been started
2 jobs for master in 8 minutes and 39 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #2739
test

00:08:39

 
  Build
skipped #2740
build
 
Name Stage Failure
failed
test Test

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warning: build failed, waiting for other jobs to finish...
error: aborting due to previous error

error: build failed
make: *** [Makefile:23: clippy] Error 101
Cleaning up file based variables
ERROR: Job failed: exit code 1