Fix issues with checking x0 when passing initial_solutions

[btjanaka]

Description

Before, passing in initial_solutions without passing x0 caused an error because x0 was still being checked despite being None. This PR fixes that bug.

We also add an initial_solutions property for retrieving the emitter's initial_solutions.

TODO

Questions

Status

• [x] I have read the guidelines in CONTRIBUTING.md
• [x] I have formatted my code using yapf
• [x] I have tested my code by running pytest
• [x] I have linted my code with pylint
• [x] I have added a one-line description of my change to the changelog in HISTORY.md
• [x] This PR is ready to go