s3: Correct emptyness check.

* src/dctc/java/com/dataiku/dctc/file/S3File.java: Confusion between
  and and or.
Test for StringReplacement.

* src/dctc/test/com/dataiku/dctc/StringReplacementTest.java: Here
* src/core/java/com/dataiku/dip/utils/StreamReplacement.java: Delete
  comments. Checked by the new test.