You're asking several separate questions that when combined seem to be a little confusing to me, but I'll try to address them all and then attempt to answer your overall problem.
Is there a way to decrypt a NULL text?
If something is NULL, that means there is nothing stored there. It's distinct from storing an empty string or a 0, because if you're storing a price list for instance, a 0 means the price is zero, but a NULL value means the price hasn't been set. In some cases, it doesn't make sense to allow a NULL (for instance, in a user registration page you might need an email address as a unique identifier, in which case a NULL email address would be problematic). There's no encryption involved with inserting a NULL value. When defining or editing a column, you can tell MySQL/MariaDB whether to allow NULL values.
I need to get access at the text of every post they upload via phpmyadmin
Your users should not be using phpMyAdmin directly. You can have them submit information via WordPress or some other software, or even a custom form, but phpMyAdmin is not mean for end users to send you data.
because I want to connect the website with an android app.
The usual way of getting an app to connect to the database that backs a website is to expose an API on the web server that the app can access. You won't be able to easily access the database directly from the Android app.
What I can't figure out is why the text field is shown as NULL but in the site it appears correctly.
The image you posted is of the phpMyAdmin Insert tab, where you add new data; if you go to the Browse tab you'll see your "Hello World!" text
So, if I try to synchronize these data with SQLite will it pass the data or will it turn NULL?
Why are you trying to synchronize your database to SQLite? How are you going to do that? I think you should pick one database and stick to that rather than trying to synchronize between two systems that have rather different features. Whether it will properly handle NULL values will depend on how you're handling the synchronization. SQLite does allow NULL values, but whether it will cleanly synchronize will depend more on how you're linking the two systems than on the capabilities of each individual database system.
Now I think you need to look at what you're trying to accomplish here, because the complexity of synchronizing the two databases plus trying to connect directly to MySQL from an Android app means to me that whatever you're working on is currently pretty fragile and probably some parts should be rethought.
NULLto the original text.