[FLINK-9577] Divide-by-zero in PageRank

[yanghua]

What is the purpose of the change

This pull request fixed a divide-by-zero error in PageRank

Brief change log

• Add a judgement to avoid divide-by-zero error

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

[zentol]

How have you verified that this does not affect the semantics of the algorithm?

[yanghua]

@zentol the field vertexCount and uniformlyDistributedScore been initialized in open function. if vertexCount was set to default value (0). I think uniformlyDistributedScore keeps default value is OK, otherwise it would throw an error. if vertexCount is not zero, the uniformlyDistributedScore would be calculated accurately.

[greghogan]

We do test for this in PageRankTest.testWithEmptyGraphWithoutVertices.

Divide-by-zero is only an error for integer division and here were are operating as double.