Fixing argument 2 passed to Safe\openssl_sign() must be of the type string, null given

[moufmouf]

See #86.

[moufmouf]

This PR actually fixes all the reference parameters that are used to "write only" by making these parameters nullable.

[coveralls]

Pull Request Test Coverage Report for Build 159

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 27 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+1.1%) to 35.436%

---

Files with Coverage Reduction	New Missed Lines	%
src/PhpStanFunctions/PhpStanParameter.php	4	82.61%
src/Parameter.php	23	49.02%

Totals
Change from base Build 156:	1.1%
Covered Lines:	191
Relevant Lines:	539

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 159

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 27 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+1.1%) to 35.436%

---

Files with Coverage Reduction	New Missed Lines	%
src/PhpStanFunctions/PhpStanParameter.php	4	82.61%
src/Parameter.php	23	49.02%

Totals
Change from base Build 156:	1.1%
Covered Lines:	191
Relevant Lines:	539

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 159

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 27 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+1.1%) to 35.436%

---

Files with Coverage Reduction	New Missed Lines	%
src/PhpStanFunctions/PhpStanParameter.php	4	82.61%
src/Parameter.php	23	49.02%

Totals
Change from base Build 156:	1.1%
Covered Lines:	191
Relevant Lines:	539

---

💛 - Coveralls