m/u: SQLite journaling defaults if not explicit

We do leverage the SQLite defaults if we don't override it explicitely in
munin config. This ensure that the safest default for the current
platform is used as the SQLite package should be trusted on it.

lib/Munin/Master/Update.pm

@@ -87,9 +87,13 @@ sub get_dbh {
 	INFO 'get_dbh: $dbh->{Driver}->{Name} = ' . $dbh->{Driver}->{Name} . ($is_read_only ? "(ro)" : "(rw)");
 
 	# Sets some session vars
-	my $db_journal_mode = $ENV{MUNIN_DB_JOURNAL_MODE} || $config->{db_journal_mode} || "TRUNCATE";
-	$dbh->do("PRAGMA journal_mode=$db_journal_mode;") if $db_driver eq "SQLite";
-	DEBUG "get_dbh: PRAGMA journal_mode=$db_journal_mode;" if $db_driver eq "SQLite";
+
+	# db_journal_mode is only set explicitely. Otherwise use the platform SQLite default
+	my $db_journal_mode = $ENV{MUNIN_DB_JOURNAL_MODE} || $config->{db_journal_mode};
+	if ($db_journal_mode) {
+		$dbh->do("PRAGMA journal_mode=$db_journal_mode;") if $db_driver eq "SQLite";
+		DEBUG "get_dbh: PRAGMA journal_mode=$db_journal_mode;" if $db_driver eq "SQLite";
+	}
 
 	my $db_synchronous_mode = $ENV{MUNIN_DB_SYNCHRONOUS_MODE} || $config->{db_synchronous_mode} || "OFF";
 	$dbh->do("PRAGMA main.synchronous=$db_synchronous_mode;") if $db_driver eq "SQLite";