Don't return NULL on 0 length input

A 0 length string is still a valid input and should be treated as such, a NULL return should be reserved for when errors occur during line editing or EOF is reached.

Merges https://github.com/espressif/esp-idf/pull/4926

components/console/linenoise/linenoise.c

@@ -979,11 +979,11 @@ char *linenoise(const char *prompt) {
     } else {
         count = linenoiseDumb(buf, LINENOISE_MAX_LINE, prompt);
     }
-    if (count > 0) {
+    if (count >= 0) {
         sanitize(buf);
         count = strlen(buf);
     }
-    if (count <= 0) {
+    if (count < 0) {
         free(buf);
         return NULL;
     }