Initialize keyword to default value

[Hitogoroshi]

When the method: OIFitsChecker.isInspectRules() has been added in the code to catch failures, we can go into methods with null values.
To prevent this problem a number of checks (on values that can be null) have been created:

if (value == null) {
}

In this case on FitsImageLoader, we propose to initialize the values to a default value so that it can not be null anymore and ensure all mandatory keywords in memory have a not null value (getters should never return null)

[bourgesl]

Please indicate which & where checks were introduced + which keywords (type ?) are concerned, only mandatory keywords ?

[Hitogoroshi]

In the case where isInspectRules () is true all the keywords are concerned.
Normally the flag hasKeyword = header.containsKey(name) checks for the presence of the keyword in the header.

Add a method so that mandatory keywords can never be null.