Fix typo in initial schema

sql/schema_version_1.sql

@@ -43,8 +43,8 @@ create table feeds (
     feed_url text not null,
     site_url text not null,
     checked_at timestamp with time zone default now(),
-    etag_header text,
-    last_modified_header text,
+    etag_header text default '',
+    last_modified_header text default '',
     parsing_error_msg text default '',
     parsing_error_count int default 0,
     primary key (id),

sql/sql.go

 // Code generated by go generate; DO NOT EDIT.
-// 2017-12-22 11:25:01.937552528 -0800 PST m=+0.002520290
+// 2017-12-22 16:35:30.206583656 -0800 PST m=+0.002482805
 
 package sql
 
@@ -49,8 +49,8 @@ create table feeds (
     feed_url text not null,
     site_url text not null,
     checked_at timestamp with time zone default now(),
-    etag_header text,
-    last_modified_header text,
+    etag_header text default '',
+    last_modified_header text default '',
     parsing_error_msg text default '',
     parsing_error_count int default 0,
     primary key (id),
@@ -162,7 +162,7 @@ alter table users add column entry_direction entry_sorting_direction default 'as
 }
 
 var SqlMapChecksums = map[string]string{
-	"schema_version_1":  "7be580fc8a93db5da54b2f6e87019803c33b0b0c28482c7af80cef873bdac4e2",
+	"schema_version_1":  "00b2fa9e945565625c93ef9d4242a8b6583dc3cd7edf38d2fc95c0f3f7b926ae",
 	"schema_version_10": "8faf15ddeff7c8cc305e66218face11ed92b97df2bdc2d0d7944d61441656795",
 	"schema_version_11": "dc5bbc302e01e425b49c48ddcd8e29e3ab2bb8e73a6cd1858a6ba9fbec0b5243",
 	"schema_version_12": "a95abab6cdf64811fc744abd37457e2928939d999c5ef00d2bdd9398e16f32fb",