Force a prompt if PS1 is empty entering a container.

[dtrudg]

Description of the Pull Request (PR):

If a container without a .singularity.d is run then there is no
environment file to set a custom PS1.

If additionally singularity is called through a script, which will reset
PS1 to empty, then a confusing silent prompt will result.

In our action script force PS1 to a default if it is empty.

This results in the example given in #5340 producing a prompt as expected:

02:31 PM $ ./dosing exec centos7-sandbox /bin/bash
Singularity> 

This fixes or addresses the following GitHub issues:

• Fixes: Default PS1 missing in 3.6.0 if run through a bash script and no .singularity.d in image #5340

Before submitting a PR, make sure you have done the following:

• Read the Guidelines for Contributing, and this PR conforms to the stated requirements.
• Added changes to the CHANGELOG if necessary according to the Contribution Guidelines
• Added tests to validate this PR and tested this PR locally with a make testall
• Based this PR against the appropriate branch according to the Contribution Guidelines
• Added myself as a contributor to the Contributors File

Attn: @singularity-maintainers

[DrDaveD]

I tested it and it fixed the problem for me.