[#585] fix: avoid unbound variable errors in start-shuffle-server.sh

[zuston]

What changes were proposed in this pull request?

If unset the var of MAX_DIRECT_MEMORY_SIZE, the error of unbound variable will be thrown when executing the start-shuffle-server.sh

Why are the changes needed?

Use the ${xxx:-} to check its existence. refer to : https://bnikolic.co.uk/blog/bash-unbound-variable

Does this PR introduce any user-facing change?

No.

How was this patch tested?

Tested in production env

[zuston]

cc @jerqi

[codecov-commenter]

Codecov Report

Merging #696 (e3f0de8) into master (00454c1) will increase coverage by 2.26%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #696      +/-   ##
============================================
+ Coverage     60.80%   63.06%   +2.26%     
- Complexity     1840     1841       +1     
============================================
  Files           221      207      -14     
  Lines         12648    10684    -1964     
  Branches       1062     1062              
============================================
- Hits           7690     6738     -952     
+ Misses         4552     3601     -951     
+ Partials        406      345      -61     

Impacted Files	Coverage Δ
deploy/kubernetes/operator/pkg/utils/rss.go
...pkg/controller/sync/shuffleserver/shuffleserver.go
...tor/pkg/controller/sync/coordinator/coordinator.go
deploy/kubernetes/operator/pkg/webhook/manager.go
deploy/kubernetes/operator/pkg/utils/config.go
...bernetes/operator/pkg/controller/controller/rss.go
...y/kubernetes/operator/pkg/webhook/inspector/pod.go
...rnetes/operator/pkg/webhook/inspector/inspector.go
...oy/kubernetes/operator/pkg/utils/shufflerserver.go
deploy/kubernetes/operator/pkg/utils/util.go
... and 5 more

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[jerqi]

LGTM

[jerqi]

Merged . Thanks @zuston