Remove axis label with NULL in create.boxplot

[stefaneng]

Description

Make behavior of axis.lab = NULL for create.boxplot consistent with other functions.

Closes #62

Checklist

• This PR does NOT contain PHI or germline genetic data. A repo may need to be deleted if such data is uploaded. Disclosing PHI is a major problem.
• This PR does NOT contain molecular files, compressed files, output files such as images (e.g. .png, .jpeg), .pdf, .RData, .xlsx, .doc, .ppt, or other non-plain-text files. To automatically exclude such files using a .gitignore file, see here for example.
• I have read the code review guidelines and the code review best practice on GitHub check-list.
• I have set up or verified the main branch protection rule following the github standards before opening this pull request.
• The name of the branch is meaningful and well formatted following the standards, using [AD_username (or 5 letters of AD if AD is too long)]-[brief_description_of_branch].
• I have added the major changes included in this pull request to the CHANGELOG.md under the next release version or unreleased, and updated the date.

Example

library(BoutrosLab.plotting.general, quietly = T, warn.conflicts = F);
set.seed(10);
simple.data <- data.frame(
    x = rnorm(100),
    y = sample(1:5, replace = T, size = 100)
    );

create.boxplot(
    formula = y ~ x,
    data = simple.data,
    yaxis.lab = NULL
    );

create.boxplot(
    formula = y ~ x,
    data = simple.data,
    xaxis.lab = NULL
    );

^{Created on 2023-01-09 with reprex v2.0.2}