Fix MISRA C++ 2023 rule 7.0.3 "The numerical value of a character shall not be used"

[mohammedRafeeque]

Zserio version and language Zserio: latest from master Commit date: 6 days ago (6/4/2024 7:12:28 PM) Commit hash: https://github.com/ndsev/zserio/commit/b8cc6c0df51cfa60740421c719f3dabfd88e0fb7

Describe the bug 2 Misra warnings are getting reported for rule MISRACPP2023-7_0_3-a

1. for (char character : value)
2. m_buffer.at(m_len) = 0;

SNo	Rule	Severity	Message	Filename	Linenumber
1298	MISRACPP2023-7_0_3-a	2	The plain char type shall be initialized only by character values	/zserio_runtime/zserio/JsonEncoder.cpp	55
1244	MISRACPP2023-7_0_3-a	2	To the plain char type assign only character values	/zserio_runtime/zserio/CppRuntimeException.cpp	42

Please check possibility of fixing the same.

Steps to reproduce the behavior: SCA run using MISRA 2023 configuration.

Expected behavior No SCA warnings for MISRA 2023 ruleset

Additional context N.A

@Roland-Homeier @mikir FYI.