This is a follow-up to JENKINS-27349.

To reproduce:

• A parametrized build with parameter BRANCH_TO_BUILD, with default value "" (or a last execution with BRANCH_TO_BUILD="" until JENKINS-27349 is fixed)
• Git SCM with branch specifier "${BRANCH_TO_BUILD}"
• SCM polling enabled

Expected:

• Should poll all branches, same as when you set the branchspec to "" directly, without parameters.

Actual:

• Nothing is matched

Workaround:

• Set the default value to "**" instead of ""