Add compare-to-empty-string example (#6861)

Author: harupy

doc/data/messages/c/compare-to-empty-string/bad.py

@@ -0,0 +1,8 @@
+x = ""
+y = "hello"
+
+if x == "":  # [compare-to-empty-string]
+    print("x is an empty string")
+
+if y != "":  # [compare-to-empty-string]
+    print("y is not an empty string")

doc/data/messages/c/compare-to-empty-string/details.rst

@@ -1 +1,8 @@
-# This is a placeholder for correct code for this message.
+x = ""
+y = "hello"
+
+if not x:
+    print("x is an empty string")
+
+if y:
+    print("y is not an empty string")

doc/data/messages/c/compare-to-empty-string/good.py

@@ -0,0 +1,2 @@
+[main]
+load-plugins=pylint.extensions.emptystring