As I understand it, the problem is that you have not been responding promptly and at length to her texts, so she now believes that you don't want them. However, you've been using her texts to maintain closeness when you do pick up your phone and check. Essentially her texts work for you, as you get what you need from her. She is telling you that she is not getting what she needs from your texting relationship.
To answer your literal question, when you go to check texts and don't find any, text her with something like (but in your words not mine):
Hey, I just finished [whatever you were doing].
Try to share at least one detail about what you were doing. It doesn't have to be deep and meaningful. You are signalling that you want her to be included in your life.
I went to check texts because I miss you, but nothing :(
Text me when you get a chance, because reading your texts is part of what gets me through the day.
This communicates that you are thinking about her. It takes the effort of sharing something about you. It indicates that you were busy. It expresses that you like getting texts from her and it makes you sad when you don't.
It shares how you feel. In absence of such communication she has been making her own guesses. Don't leave her guessing. Tell her.
Addressing the underlying problem
Also, at least once a day, text her
Just saw [...] and it reminded me of you.
Obviously replace [...] with something that actually did remind you of her. This can also be a good use of the phone camera. Try to keep things upbeat rather than critical. Something that might be funny if discussed in person can come off as harsh over text. And she has already started expressing insecurity, so things that can be taken two ways will probably be taken the negative way.
There are some sociologists who believe that men and women use conversation differently. Men think of it as a way to communicate things. So men who don't have anything to communicate don't text. Women use conversation to maintain connection. From that perspective, she seems to be indicating that she is not feeling a connection from you.
If she is texting a lot and you are texting a little, take time to send more texts. Perhaps you don't have time to reply during the day. If so, set up a time when you do have time to reply and reread her texts. Reply then, preferably in kind. For example, if she's sharing details of her day, share details of yours. If she's talking about her family, talk about yours. She is telling you the kind of things she finds interesting. Keep her interested by talking about those kinds of things. Also respond to what she's saying. Commiserate when she's complaining; cheer when she's happy. Show that you're paying attention.
If the purpose is connection, your texts do not have to communicate deep and meaningful things. Say you missed your bus and have to wait for twenty minutes for the next one. That's a great time to text and share some details of your life. Had any good meals lately? Remember that shared meal with ...? Were you woken up by some odd sound? Why'd you miss the bus? You said "miss" does that make you think about missing her?
Look for things that you'd like to share with her if she was there. It could be a place to eat breakfast, a music store, a park, or whatever. Take a picture and send it with a note. If you don't have time to write the note, take the picture and save it for later.
Tell her about times when you wish she was there. Tell her why you wished she was there. Did you want to hold her hand? Share sarcastic observations about other people there? What did you enjoy doing with her when you were physically together? Talk about those things. You may want to keep it PG in texts in case someone else might read her phone. But other than that, things are wide open.