sh: Do not backspace if command is empty

apps/src/sh.c

@@ -139,11 +139,33 @@ void command_concat(command* cmd, const char* str)
 {
     while (*str)
     {
+        if (*str == '\b')
+        {
+            if (cmd->size == 0)
+            {
+                str++;
+                continue;
+            }
             putchar(*str);
-        if (*str == '\b') { command_pop(cmd); }
+            command_pop(cmd);
-        else if (*str == '\n') { command_execute(cmd); }
+        }
+        else if (*str == '\n')
+        {
+            if (cmd->size == 0)
+            {
+                putchar(*str);
+                show_prompt();
+                str++;
+                continue;
+            }
+            putchar(*str);
+            command_execute(cmd);
+        }
         else
+        {
+            putchar(*str);
             command_push(cmd, *str);
+        }
         str++;
     }
 }