【show backends】'DataUsedCapacity' display needs to be optimized

[lvchenyang-maker]

Steps to reproduce the behavior (Required)

1. Deploy a new cluster

show backends;

Expected behavior (Required)

'DataUsedCapacity'=0.0

Real behavior (Required)

'DataUsedCapacity'=.000

StarRocks version (Required)

• You can get the StarRocks version by executing SQL select current_version()

mysql> select current_version();
+-------------------+
| current_version() |
+-------------------+
| 2.0.1 f0de9ec     |
+-------------------+
1 row in set (0.04 sec)

[lvchenyang-maker]

@mergify backport branch-2.4