feat: Changed assert to Preconditions check

[jiangmichaellll]

As titled.

[codecov]

Codecov Report

Merging #457 (a97ba26) into master (d1c2a24) will increase coverage by 0.32%.
The diff coverage is 80.00%.

@@             Coverage Diff              @@
##             master     #457      +/-   ##
============================================
+ Coverage     71.14%   71.47%   +0.32%     
+ Complexity      917      913       -4     
============================================
  Files           168      168              
  Lines          4873     4876       +3     
  Branches        245      239       -6     
============================================
+ Hits           3467     3485      +18     
- Misses         1268     1269       +1     
+ Partials        138      122      -16     

Impacted Files	Coverage Δ	Complexity Δ
.../pubsublite/spark/PslContinuousInputPartition.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...blite/spark/PslContinuousInputPartitionReader.java	70.37% <50.00%> (+6.08%)	5.00 <1.00> (ø)
...blite/spark/PslMicroBatchInputPartitionReader.java	67.64% <50.00%> (+4.78%)	7.00 <1.00> (ø)
...le/cloud/pubsublite/spark/PslMicroBatchReader.java	88.88% <87.50%> (+7.93%)	11.00 <4.00> (-1.00)	⬆️
...le/cloud/pubsublite/spark/PslContinuousReader.java	61.90% <100.00%> (+11.90%)	7.00 <2.00> (-1.00)	⬆️
...m/google/cloud/pubsublite/spark/PslSparkUtils.java	95.00% <100.00%> (+1.66%)	12.00 <0.00> (-1.00)	⬆️
...ogle/cloud/pubsublite/spark/SparkSourceOffset.java	91.48% <100.00%> (+6.38%)	16.00 <1.00> (-1.00)	⬆️
... and 4 more

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