[SQL] Remove unnecessary case in HiveContext.toHiveString

[scwf]

a follow up of #3547
/cc @marmbrus

[SparkQA]

Test build #24060 has started for PR 3563 at commit 9395661.

• This patch merges cleanly.

[SparkQA]

Test build #24060 has finished for PR 3563 at commit 9395661.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • public class NullType extends DataType

[AmplabJenkins]

Test PASSed.
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Test PASSed.

[scwf]

@marmbrus this should be ready to go:)

[liancheng]

LGTM, thanks!

[marmbrus]

Thanks! Merged to master.