Don't segfault in ArraySchema::dump

[davisp]

There was no check for whether an enumeration was loaded before attempting to dump it during a call to ArraySchema::dump. Obviously, dereferencing nullptr is bad so this avoids doing that.

The new output now shows the following when a Enumeration isn't loaded:

### Attribute ###
- Name: a
- Type: INT32
- Nullable: false
- Cell val num: 1
- Filters: 0
- Fill value: -2147483648
- Enumeration name: flintstones

### Enumeration ###
- Name: flintstones
- Loaded: false

And after calling load_all_enumerations:

### Attribute ###
- Name: a
- Type: INT32
- Nullable: false
- Cell val num: 1
- Filters: 0
- Fill value: -2147483648
- Enumeration name: flintstones

### Enumeration ###
- Name: flintstones
- Loaded: true
- Type: STRING_ASCII
- Cell Val Num: 4294967295
- Ordered: false
- Element Count: 4

---

TYPE: BUG
DESC: Don't segfault in ArraySchema::dump

[shortcut-integration]

This pull request has been linked to Shortcut Story #38300: Array schema dump segfaults if enumerations are not loaded..