check correct usage of `LENGTH`/`length` in C code

[ben-schwen]

          Are any of the other >200 places we use `LENGTH()` worth changing too? (maybe glance through now if anything looks like an obvious fix to put in the patch release, and file a follow-up to go through more carefully in the future)

Originally posted by @MichaelChirico in #6875 (comment)

[ben-schwen]

I have checked all mentions of LENGTH() in our code. Here are a few observations:

• We have several mentions of when to use length(x) over LENGTH(x), e.g. when x can be NULL
• In most places we safe-guard by checking the type, e.g. isString(), isInteger(), etc.
• Sometimes we safe-guard by checking length(x) before using LENGTH(x).