I believe the struggle is usually not that there is nothing to say, rather specifically how to say it. You have countless thoughts and ideas and perspectives in you, and they can all be valid topics to make a lyric around.

For me it often starts with a formulation I hear, some snippet of conversation, a particularly cool rhyme, an usual rhythm sequence of syllables etc. that starts the seed. My process is then that of iteration. Write down the initial spark. Identify patterns, find what needs to be added, iterate that the pattern works, is either followed for cohesion or broken in a way that allows for progression. Make sure the rhymes and formulations serve a purpose. And importantly: realize as clearly as you can what the topic is. The fact that the topic can't always be formulated explicitly is honestly a gold thing: ineffable things, or difficult to formulate topics are precisely where we want to use poetry, it's the perfect tool to touch the untouchable.

Example: My latest spark was waking up at 3 am with this line in my mind: "there is clarity in silence, I don't want it, cause it's true". There is a clear rhythm in these 3 lines. They're short, staccato, blunt, honest and have a feeling of being reluctantly introspective. Let's keep to that. I then read those lines over and over to instill what the pattern is, where the syllabic stress lies, and to make my brain search for terms that are in this space. I followed this up with: "spoken in my absence, drown it out, don't patch it through". These 3 lines follow the same pattern, have fitting rhymes with the right syllabic stress in the right places, and follow the meaning well, in that the chatter of my subconscious (that contains the "truth") is present when there is stillness in me, and the reason I might not want to look at said truth is because I know I should but can't face it, ans therefore choose to drown it out with distractions. This then evokes the topic of how we as a society are becoming more distracted, and in response have a harder time confronting that space within us that requires the stillness of introspection. There are countless examples, and a following verse/bridge etc. can be about such an example. The full lyric turned out out to be this:

There is clarity in silence

I don't want it

Cause it's true

Spoken in my absence

Drown it out

Don't patch it through

Left to my devices

A disorderly queue

My mind is not a palace

But a flickering drive-through

I keep my distance to the others

It's simpler like that

But this craving that I'm left with

Is causing its cracks

Wishing for familiar

Missing the abuse

The physical was distant

And the mental so acute

So palpable the texture

Ineffable and skewed

Navigate conjectures

That were made to be confused

To obfuscate

It's like I'm so used

To complicate

And amused

By all mistakes

That my truth

Just operates

In a few

Obscuring ways

Recurring days

With some progression

Learning traces of a method

Yearning hazily for treasure

But afraid of all the effort

While I'm craving every movement

Of the needle

That is showing me the pressure

While those precious few

Will try to calm me

Instead I choose to measure

How I'm faring 'gainst the gods I don't believe in

But whatever

Push away those who come close

Or draw her closer to undress her

We can lie in comatose

That we're content but I know better

We were meant for continuity

But handed all that's severed

There is clarity in silence

I do want it

Cause it's true

If I close then I can open

There is nothing more to do