[ci] fixed cpplint warnings about braces

[jameslamb]

This PR eliminates 5 of the 49 remaining cpplint issues, noted in #1990 --> #1990 (comment)

src/io/file_io.cpp:173:  If an else has a brace on one side, it should have it on both  [readability/braces] [5]
src/io/file_io.cpp:186:  If an else has a brace on one side, it should have it on both  [readability/braces] [5]
src/io/file_io.cpp:197:  If an else has a brace on one side, it should have it on both  [readability/braces] [5]
src/io/file_io.cpp:197:  If/else bodies with multiple statements require braces  [readability/braces] [4]
Done processing src/io/file_io.cpp
...
include/LightGBM/tree.h:154:  Line ends in whitespace.  Consider deleting these extra spaces.  [whitespace/end_of_line] [4]

4 of the 5 relate to handling of HDFS files in src/io/file_io.cpp. Instead of using an if-else construct, I think we can get around the linting issue by just relying on the fact that the affected functions already use multiple return statements.

Looks like the relevant lines were introduced in #2769 , @guolinke

[github-actions]

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