fix: fix taskmanager config handle null string

[tobegit3hub]

• Resolve Fail to start taskmanager server when not configuring hadoop.conf.dir #3596 .

[codecov]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (825d155) 75.33% compared to head (e311d0a) 75.33%.

Files	Patch %	Lines
...openmldb/taskmanager/config/TaskManagerConfig.java	0.00%	9 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #3597      +/-   ##
============================================
- Coverage     75.33%   75.33%   -0.01%     
  Complexity      635      635              
============================================
  Files           727      727              
  Lines        130673   130673              
  Branches       1317     1317              
============================================
- Hits          98446    98443       -3     
- Misses        31936    31939       +3     
  Partials        291      291              

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[tobegit3hub]

# Server Config
server.host=0.0.0.0
server.port=9902
server.worker_threads=4
server.io_threads=4
server.channel_keep_alive_time=1800
prefetch.jobid.num=1
job.log.path=./logs/
external.function.dir=./udf/
track.unfinished.jobs=true
job.tracker.interval=30

# OpenMLDB Config
zookeeper.cluster=0.0.0.0:2181
zookeeper.root_path=/openmldb
zookeeper.session_timeout=5000
zookeeper.connection_timeout=5000
zookeeper.max_retries=10
zookeeper.base_sleep_time=1000
zookeeper.max_connect_waitTime=30000

# Spark Config
spark.master=local[*]
offline.data.prefix=file:///tmp/openmldb_offline_storage/