kconfiglib: check for empty strings on macro expansion

[leandrolanzieri]

During macro expansion, bare macros on a line are accepted by the parser as long as they resolve to blank strings. The problem is that the script is currently checking using isspace, so it's actually not checking for blank strings.

This causes the parsing to fail when a macro is the last line of a file, and no newline character is added afterwards. This patch adds a check for the string itself being empty. It can be tested, for instance, by applying the following and running the tests:

diff --git a/tests/Kpreprocess b/tests/Kpreprocess
index 283a988..a30ba74 100644
--- a/tests/Kpreprocess
+++ b/tests/Kpreprocess
@@ -149,3 +149,5 @@ env_ref_3 += $(ENV_4)
 $(warning-if,$(ENV_5),$(ENV_UNDEFINED))
 source "$(ENV_6)"
 env_ref_4 = $(ENV_7)  # Never evaluated
+
+$(info,Information message)
\ No newline at end of file