Empty false condition for builder dyn_if

[dessalines]

I often only want to show something in the true condition, but the dyn_if function has three arguments:

• Condition
• True case
• False case

The false case should probably either be an Option type, or there should be a helper function that is just empty html.

Currently I'm doing:

.dyn_if(|| condition, || ..., || fragment([]) as a workaround.