Avoid adding monitor sink when not intended

[PhilipFackler]

Description

This fixes a bug in the dynamic simulation app that was adding a console sink to the monitor when no output file is specified.

Proposed changes

Only add the output file from the solver json file if it is non-empty.

Checklist

• All tests pass.
• Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra.
• The new code follows GridKit™ style guidelines.
• [N/A] There are unit tests for the new code.
• [N/A] The new code is documented.
• The feature branch is rebased with respect to the target branch.
• [N/A] I have updated CHANGELOG.md to reflect the changes in this PR. If this is a minor PR that is part of a larger fix already included in the file, state so.