`msqrobAggregate` cannot handle models with only fixed effects

[cvanderaa]

Eg:

library(msqrob2)
data(pe)
pe <- msqrobAggregate(
    pe, i = "peptide", fcol = "Proteins", formula = ~ 1 + condition
)

Error in .local(object, ...) : Formula contains no random effects.

We should keep in mind that a model with only fixed effects makes no sense in most cases for msqrobAggregate and may hint towards a wrong model specification. However, I encounter the issue when refitting the models for one-hit wonder proteins (cf #68).