cli: Fix crash on missing args (#94)

Fix crash that occurs when running vsql cli without a .vsql file cmd argument.

cmd/vsql.v

@@ -8,6 +8,8 @@ import vsql
 fn main() {
 	mut cmd := cli.Command{
 		name: 'vsql'
+		usage: '<file>'
+		required_args: 1
 		description: 'vsql is a single-file SQL database written in V'
 		execute: cli_command
 	}
@@ -61,23 +63,27 @@ fn cli_command(cmd cli.Command) ? {
 	print_version()
 
 	mut db := vsql.open(cmd.args[0])?
+
 	for {
 		print('vsql> ')
 		query := os.get_line()
 
-		start := time.ticks()
-		result := db.query(query)?
-		for row in result {
-			for column in result.columns {
-				print('$column.name: ${row.get_string(column.name)} ')
+		if query != "" {
+			start := time.ticks()
+			result := db.query(query)?
+			for row in result {
+				for column in result.columns {
+					print('$column.name: ${row.get_string(column.name)} ')
+				}
+			}
+
+			if result.rows.len > 0 {
+				println('')
 			}
-		}
 
-		if result.rows.len > 0 {
-			println('')
+			println('$result.rows.len ${vsql.pluralize(result.rows.len, 'row')} (${time.ticks() - start} ms)')
 		}
 
-		println('$result.rows.len ${vsql.pluralize(result.rows.len, 'row')} (${time.ticks() - start} ms)')
 		println('')
 	}
 }