patch for the isolated nodes bug (#36)

[masato-fuk]

Before submitting, please check the following:

• Make sure you have tests for the new code and that test passes (run pytest)
• format added code and tests by black -l 120 <filename>

Then, please fill in below:

Context (if applicable):

na

Description of the change:

Added a missing conditional branch(graphix.pattern l1139-1140). Added a new test for the removal of isolated nodes.

Related issue:

36

also see that checks (github actions) pass. If lint check keeps failing, try installing black==22.8.0 as behavior seems to vary across versions.