Fix warning

contrib/block_time.pl

@@ -7,7 +7,7 @@ use strict;
 
 # Given a block time, estimate when it will happen
 my $block      = shift || die "Usage: $0 123";
-my $coin       = shift;
+my $coin       = shift || '';
 my $hush       = "./src/hush-cli";
 unless (-e $hush) {
     die "$hush does not exist, aborting";