Fix findbugs violations in restconf-nb-rfc8040

Fix findbugs violations in restconf-nb-rfc8040

- Possible null pointer dereference
- Method call passes null for non-null parameter
- Nullcheck of value previously dereferenced
- Consider using Locale parameterized version of invoked method
- Reliance on default encoding
- Inconsistent synchronization
- Inefficient use of keySet iterator instead of entrySet iterator
- Should be a static inner class
- Useless assignment in return statement
- Method uses the same code for two branches
- Method uses the same code for two switch clauses
- Write to static field from instance method
- Redundant nullcheck of value known to be non-null
- Exception is caught when Exception is not thrown
- Useless control flow

Change-Id: I217a81d9dc487fe7f8e62a9c882c7335815ae052
Signed-off-by: Tom Pantelis <tompantelis@gmail.com>