libc/puts: newline was omitted for empty string

[liamHowatt]

puts(""); did not print a newline. The standard
behavior is to print a newline even if the string
is empty.

Note: Please adhere to Contributing Guidelines.

Summary

From man 3 puts on Linux:

puts() writes the string s and a trailing newline to stdout.

Impact

Tests that have puts("") may fail if they compare stdout against an expected result.

Testing

sim:nsh

puts("");

Should print a newline to stdout. Currently, nothing is printed.

Edit: Only an issue for the CONFIG_FILE_STREAM implementation. When disabled, a newline is included correctly.