Add ability to start tui with no input file

tui_main.c

@@ -13,7 +13,7 @@
 static void usage() {
   // clang-format off
   fprintf(stderr,
-      "Usage: ui [options] infile\n\n"
+      "Usage: ui [options] [file]\n\n"
       "Options:\n"
       "    -h or --help  Print this message and exit.\n"
       );
@@ -247,13 +247,9 @@ int main(int argc, char** argv) {
     fprintf(stderr, "Expected only 1 file argument.\n");
     return 1;
   }
-  if (input_file == NULL) {
-    fprintf(stderr, "No input file.\n");
-    usage();
-    return 1;
-  }
 
   Field field;
+  if (input_file) {
     field_init(&field);
     Field_load_error fle = field_load_file(input_file, &field);
     if (fle != Field_load_error_ok) {
@@ -281,6 +277,10 @@ int main(int argc, char** argv) {
       fprintf(stderr, "File load error: %s.\n", errstr);
       return 1;
     }
+  } else {
+    input_file = "unnamed";
+    field_init_fill(&field, 25, 57, '.');
+  }
   Markmap_reusable markmap_r;
   markmap_reusable_init(&markmap_r);
   markmap_reusable_ensure_size(&markmap_r, field.height, field.width);