You can adjust your Sensitivity, to Medium or Low and check the scanning options in the list, choosing the level that best suits your needs. Or you could set it to custom level and adjust the Heuristic settings so as not to check the subject, however, I thought this was off by default (see image), set slider to Custom and click the Customize button in the resident task settings screen.
Because avast uses a proxy to scan the emails before they arrive in your inbox that may be a little more difficult. If you were even able to do this (in the home version, you don't mention what version you use), it would mean that there is a potentially infected email in the inbox. Any subsequent on-demand scan could also find this and there may be a risk that the entire email inbox (.pst) file is removed rather than the infected email.
Whilst replying I have also sent myself a test email with a blank subject and it has been received without an alert from avast and my Sensitivity setting is High, using OE 6. So I'm not sure why/how mine and your settings differ. I don't use MS Outlook or the Outlook/Exchange plug-in so I don't know if the comparison is valid.