Update osimC3D to deal with empty tables

[jimmyDunne]

Fixes issue seen in #2642

Brief summary of changes

• Added flag and subsequent checks in functions to see if there are empty marker or force tables
• updated indents in some sections

Testing I've completed

Tested using some c3ds with empty force or marker tables. Passed ctest, locally.

Looking for feedback on...

• flags and calls look okay? Is there a better way of doing this?

CHANGELOG.md (choose one)

• no need to update because this fixes a bug that we haven't see in the wild yet (but is expected)

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This change is