[MINOR][0.7] Avoid returning null in defaultUserApps when quota file does't config user

[smallzhongfeng]

What changes were proposed in this pull request?

map.getOrDefault() => map.computeIfAbsent()

Why are the changes needed?

If do not define the user and app num in the quota configuration file, it will appear that the defaultUserApps does not have the corresponding app number for this user, because we did not include the quotaAppNum in the defaultUserApps.

before :
The log from coordinator is [ERROR] 2023-03-28 19:24:11,18 Grpc-260 AccessAppQuotaChecker check - Denied by AccessAppQuotaChecker => User: xxxx, current app num is: 3, default app num is: null.

after :
The log from coordinator is [ERROR] 2023-03-31 14:32:21,228 Grpc-240 AccessAppQuotaChecker check - Denied by AccessAppQuotaChecker => User: xxxx, current app num is: 3, default app num is: 3.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

Origin uts.

[smallzhongfeng]

PTAL. @jerqi Thanks!

[codecov-commenter]

Codecov Report

Merging #822 (48c4246) into branch-0.7 (bcb591e) will not change coverage.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##             branch-0.7     #822   +/-   ##
=============================================
  Coverage         60.78%   60.78%           
- Complexity         1792     1793    +1     
=============================================
  Files               214      214           
  Lines             12374    12374           
  Branches           1045     1045           
=============================================
  Hits               7521     7521           
  Misses             4449     4449           
  Partials            404      404           

Impacted Files	Coverage Δ
...a/org/apache/uniffle/coordinator/QuotaManager.java	85.29% <100.00%> (ø)

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[jerqi]

LGTM, thanks @smallzhongfeng merged.