Improve the use of strings in reference values & ProvenanceIR

[rbehjati]

Currently, in the verifier, the endorser, and ProvenanceIR, we assign different meanings to a nil string and an empty ("") string. This is against some best practices:

https://go.dev/doc/effective_go#allocation_new discusses zero values and "make the zero value useful".

go-style/decisions#literal-zero-value-fields asserts the readability of zero-value construction.

A counterargument

A more common pattern is to use a boolean to signal the presence or absence of a value.

Ref: Readability review on cl/536413558

CC @mariaschett