Namelist build script skips last line if it doesn't have a return on it

### Brief summary of bug

@adrifoster and I have noticed that, if you specify something in the very last line of a `user_nl_clm` file, it's not actually incorporated into the built namelist. This could quietly cause major problems if a user puts something behavior-changing there that gets ignored.

### General bug information

**CTSM version you are using:** All

**Does this bug cause significantly incorrect results in the model's science?** Potentially

**Configurations affected:** All

### Details of bug

Resolving this bug will require a fix to the build-namelist script. In the meantime, we should (a) fix the testmods that don't have a blank line at the bottom of `user_nl_clm` and (b) add a Github action that ensures new testmod `user_nl_clm` files have a blank last line.