Here's how to test whether a parameter is unset, or empty ("Null") or set with a value:
+--------------------+----------------------+-----------------+-----------------+
| | parameter | parameter | parameter |
| | Set and Not Null | Set But Null | Unset |
+--------------------+----------------------+-----------------+-----------------+
| ${parameter:-word} | substitute parameter | substitute word | substitute word |
| ${parameter-word} | substitute parameter | substitute null | substitute word |
| ${parameter:=word} | substitute parameter | assign word | assign word |
| ${parameter=word} | substitute parameter | substitute null | assign word |