Fix usage of if inside paste() which works differently from c().

[astamm]

After merging #176, package kergp was newly broken. Logs were:

• Version: 0.5.8
• GitHub: NA
• Source code: https://github.com/cran/kergp
• Date/Publication: 2024-11-19 14:40:03 UTC
• Number of recursive dependencies: 70

Run revdepcheck::revdep_details(, "kergp") for more info

Newly broken

• checking examples ... ERROR

  Running examples in ‘kergp-Ex.R’ failed
  The error most likely occurred in:
  
  > ### Name: optimMethods
  > ### Title: Optimization Methods (or Algorithms) for the 'mle' Method
  > ### Aliases: optimMethods
  > 
  > ### ** Examples
  > 
  > optimMethods()
  Error in data.frame(optimFun = c(rep("stats::optim", length(oMO)), rep("nloptr::nloptr",  : 
    arguments imply differing number of rows: 44, 43
  Calls: optimMethods -> data.frame
  Execution halted
  

It turns out that the use of if statement within the paste() function behaves differently from its use within c().