local testnet: Fix an error on startup (#2973)

## Issue Addressed

Resolves https://github.com/sigp/lighthouse/issues/2763#issuecomment-1024858187

## Proposed Changes

- Skip if the line is blank. 👌

scripts/local_testnet/kill_processes.sh

@@ -8,6 +8,9 @@ set -Eeuo pipefail
 if [ -f "$1" ]; then
   while read pid
     do
+      # handle the case of blank lines
+      [[ -n "$pid" ]] || continue
+
       echo killing $pid
       kill $pid
     done < $1