[OpenAPI Generator] Fix @Nullable property accessors

[newtork]

Currently we generate @Nonnull for all accessors in POJO model classes.

This change considers the nullable: true setting definable in specs.

Consider property foo of model Bar is...

nullable:true and required:true

• Bar foo( @Nullable foo )
• Builder foo( @Nullable foo )
• Bar setFoo( @Nullable foo )
• @Nullable getFoo()

nullable:false and required:true

• Bar foo( @Nonnull foo )
• Builder foo( @Nonnull foo )
• Bar setFoo( @Nonnull foo )
• @Nonnull getFoo()

nullable:true and required:false

• Bar foo( @Nullable foo )
• Bar setFoo( @Nullable foo )
• @Nullable getFoo()

nullable:false and required:false

• Bar foo( @Nullable foo )
• Bar setFoo( @Nullable foo )
• @Nonnull getFoo()

I also fixed a minor whitespace issue: For easier review, please hide whitespace changes.