Fix findbugs violations in restconf-nb-bierman02

Fix findbugs violations in restconf-nb-bierman02

- 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
- Inefficient use of keySet iterator instead of entrySet iterator
- Private method is never called
- Boxing/unboxing to parse a primitive
- Method concatenates strings using + in a loop
- Unchecked/unconfirmed cast of return value from method
- Method uses the same code for two switch clauses
- Parameter must be non-null but is marked as nullable
- Redundant nullcheck of value known to be non-null
- Exception is caught when Exception is not thrown
- Useless control flow

Change-Id: Ifd737d8c54532d98dd162db8d2a5915e38b13ff1
Signed-off-by: Tom Pantelis <tompantelis@gmail.com>