You could use the default-value syntax:
somecommand ${1:-foo}
The above will, as described in Bash Reference Manual – 3.5.3 Shell Parameter Expansion [emphasis mine]:
If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of parameter is substituted.
If you only want to substitute a default value if the parameter is unset (but not if it’s null, e.g. not if it’s an empty string), use this syntax instead:
somecommand ${1-foo}
Again from Bash Reference Manual – 3.5.3 Shell Parameter Expansion:
Omitting the colon results in a test only for a parameter that is unset. Put another way, if the colon is included, the operator tests for both parameter’s existence and that its value is not null; if the colon is omitted, the operator tests only for existence.