Newline not always added at the end when input has no newline

Hi,

First of all, thanks for the great tool which I have been using daily for the past 2 years! Thanks in particular for the 5.X release with the --profile black option which is very handy!

I have been observing what I think is an inconsistency in the output of isort when run with a string from stdin.
A git bisect shows that this inconsistency was introduced with commit 2ba782fb1a74294f2a67cd64fe4b6bf98dddba30

Consider the following snippet of code:

import isort

str1 = "import os\nprint(os.environ)"
str2 = "import os\ndef main():\n    print(os.environ)"
for string in [str1, str2]:
    print("Input:")
    print(repr(string))
    print("Output:")
    print(repr(isort.SortImports(file_contents=string).output))
    print("----")

When run at commit f5ef546ede9450b19d7c615ab7e7c833f88a7dcf (i.e. just before the offending commit), it prints:

Input:
'import os\nprint(os.environ)'
Output:
'import os\n\nprint(os.environ)\n'  <- newline added
----
Input:
'import os\ndef main():\n    print(os.environ)'
Output:
'import os\n\n\ndef main():\n    print(os.environ)\n' <- newline added
----

When run at the offending commit (2ba782fb1a74294f2a67cd64fe4b6bf98dddba30), it prints:

Input:
'import os\nprint(os.environ)'
Output:
'import os\n\nprint(os.environ)\n' <- newline added
----
Input:
'import os\ndef main():\n    print(os.environ)'
Output:
'import os\n\n\ndef main():\n    print(os.environ)' <- no newline added
----

I consider commit f5ef546ede9450b19d7c615ab7e7c833f88a7dcf to be consistent because it adds a newline at the end of the output in both cases.
However, commit 2ba782fb1a74294f2a67cd64fe4b6bf98dddba30 (up til the HEAD of develop as of writing) is inconsistent because it adds a newline in the str1 case, but not in the str2 case.