message feed: Fix display height of empty messages.
message feed: Fix display height of empty messages.

Ideally, empty messages are never created.  However, sometimes even
non empty messages can act as if empty, such as one consisting only
of an opening and closing blockquote tag.  This sets the
.message_content to have a minimum height equal to the line height.
This is overridden with zero (the default) in the case of collapse.

mahim97/zulip,zulip/zulip,kou/zulip,andersk/zulip,timabbott/zulip,punchagan/zulip,eeshangarg/zulip,showell/zulip,eeshangarg/zulip,rht/zulip,zulip/zulip,rishig/zulip,rishig/zulip,timabbott/zulip,dhcrzf/zulip,tommyip/zulip,dhcrzf/zulip,shubhamdhama/zulip,hackerkid/zulip,rishig/zulip,rishig/zulip,kou/zulip,eeshangarg/zulip,jackrzhang/zulip,rishig/zulip,mahim97/zulip,brainwane/zulip,synicalsyntax/zulip,tommyip/zulip,showell/zulip,rht/zulip,punchagan/zulip,showell/zulip,rht/zulip,punchagan/zulip,hackerkid/zulip,jackrzhang/zulip,punchagan/zulip,synicalsyntax/zulip,synicalsyntax/zulip,mahim97/zulip,rht/zulip,brainwane/zulip,zulip/zulip,andersk/zulip,brainwane/zulip,timabbott/zulip,kou/zulip,tommyip/zulip,synicalsyntax/zulip,tommyip/zulip,eeshangarg/zulip,kou/zulip,timabbott/zulip,rht/zulip,brainwane/zulip,kou/zulip,kou/zulip,mahim97/zulip,andersk/zulip,kou/zulip,dhcrzf/zulip,andersk/zulip,jackrzhang/zulip,hackerkid/zulip,zulip/zulip,mahim97/zulip,synicalsyntax/zulip,shubhamdhama/zulip,showell/zulip,showell/zulip,synicalsyntax/zulip,hackerkid/zulip,showell/zulip,andersk/zulip,dhcrzf/zulip,hackerkid/zulip,hackerkid/zulip,hackerkid/zulip,rishig/zulip,tommyip/zulip,andersk/zulip,rht/zulip,punchagan/zulip,jackrzhang/zulip,dhcrzf/zulip,shubhamdhama/zulip,tommyip/zulip,eeshangarg/zulip,timabbott/zulip,brainwane/zulip,dhcrzf/zulip,jackrzhang/zulip,timabbott/zulip,zulip/zulip,dhcrzf/zulip,punchagan/zulip,shubhamdhama/zulip,brainwane/zulip,jackrzhang/zulip,rht/zulip,rishig/zulip,brainwane/zulip,eeshangarg/zulip,zulip/zulip,shubhamdhama/zulip,zulip/zulip,timabbott/zulip,shubhamdhama/zulip,jackrzhang/zulip,tommyip/zulip,eeshangarg/zulip,synicalsyntax/zulip,showell/zulip,mahim97/zulip,shubhamdhama/zulip,punchagan/zulip,andersk/zulip