strbuf: allow zero-length lines

They aren't EOF.

strbuf.c

@@ -37,8 +37,6 @@ void read_line(struct strbuf *sb, FILE *fp, int term) {
 			break;
 		strbuf_add(sb, ch);
 	}
-	if (sb->len == 0)
-		sb->eof = 1;
 	strbuf_end(sb);
 }