CLEANUP: Use IST_NULL whenever possible

Refactoring performed with the following Coccinelle patch:

    @@
    @@

    - ist2(NULL, 0)
    + IST_NULL
diff --git a/src/http_fetch.c b/src/http_fetch.c
index c73546b..3d627a9 100644
--- a/src/http_fetch.c
+++ b/src/http_fetch.c
@@ -818,7 +818,7 @@
 	if (args->type == ARGT_STR) {
 		name = ist2(args->data.str.area, args->data.str.data);
 	} else {
-		name = ist2(NULL, 0);
+		name = IST_NULL;
 	}
 
 	ctx.blk = NULL;
@@ -954,7 +954,7 @@
 	if (args->type == ARGT_STR) {
 		name = ist2(args->data.str.area, args->data.str.data);
 	} else {
-		name = ist2(NULL, 0);
+		name = IST_NULL;
 	}
 
 	ctx.blk = NULL;