Division by 0 in LB stats calculations

[PhilMiller]

Describe the bug

The variance and imbalance metric calculations are prone to division by zero in the case of null contributions.

This became apparent in EMPIRE testing. It's not clear why vt's tests didn't surface this.

To Reproduce
Steps to reproduce the behavior:

1. Example/test/snippet of code that fails
2. Compiler, platform, libraries
3. Run command: number of processors, threading options, etc.
4. See error

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots
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Platform (please complete the following information):

• Supercomputer or desktop [e.g., Trinity, Sierra)
• Compiler environment [e.g., clang-3.9, gcc8]
• Libraries [e.g., mpich-2, openmpi)
• Commit ID or release of VT [e.g., vt-1.0.0]

Additional context
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