ProtoParserToolsTest.testReadString() fails if string length is zero

`ProtoParserToolsTest.testReadString()` generates a random string to test `ProtoParserTools.readString()`. Generated string length is random in interval [0, 100). This length is passed to `RandomString` utility class, which throws an exception, if the length is zero. The test should be corrected to not allow zero string lengths, and other tests that use `RandomString` should be inspected to have the same issue.