[build] When generating image version, handle case where current commit has no reachable tags

[jleveque]

- What I did

If building a SONiC image from a commit with no reachable parent tag (i.e., git describe --tags returns an empty string), sonic_get_version() would return an empty string, which would then be passed to docker tag which would fail because an empty string is not considered a valid tag.

This fix now also checks if git describe --tags returns an empty string. If so, we will create the version string based on branch and commit rather than tag.

- How to verify it

Build a SONiC image from a commit that has no reachable tag. Ensure the build succeeds and verify that the image version is based on current branch and commit.

[lguohan]

usually, to avoid empty string, we can do x"$describe" == x"$latest_tag"

[jleveque]

The convention you describe doesn't fix this issue. Prepending an "x" to the string only will prevent a bash error if either of the strings are empty (and unquoted). For example: if [ x$VAR1 == x$VAR2 ]. In this case, if VAR1 is empty and VAR2 contains "somestring", the statement would expand to if [ x == somestring ], whereas without the x's, it would expand to if [ == somestring ], which is invalid syntax and cause the script to crash. With quotes (if [ x"$VAR1" == x"$VAR2" ], this would evaluate to if [ x == xsomestring ], thus making a valid statement which would correctly return false. There is no need to prepend characters if the variables in the conditional are quoted.

In the case presented here, however, the strings are already quoted, so we don't have to worry about a crash. In fact, the issue here stems from both strings being empty. However, adding x's here doesn't solve the issue, because both latest_tag and describe are empty strings, so this comparison returns true, and will continue to return true if we prepend x's, so the result will be the same: since latest_tag is empty, sonic_get_version() would still return an empty string.

[lguohan]

use convention.