[Bug]: GCPProperties and AzureProperties throw NumberFormatException on null property values

### Apache Iceberg version

main (development)

### Query engine

Other

### Please describe the bug 🐞

`GCPProperties` and `AzureProperties` throw `NumberFormatException` when optional numeric properties exist in the properties map with `null` values.

#### Problem

In `GCPProperties.java`:
```java
if (properties.containsKey(GCS_CHANNEL_READ_CHUNK_SIZE)) {
gcsChannelReadChunkSize = Integer.parseInt(properties.get(GCS_CHANNEL_READ_CHUNK_SIZE));
}

if (properties.containsKey(GCS_CHANNEL_WRITE_CHUNK_SIZE)) {
gcsChannelWriteChunkSize = Integer.parseInt(properties.get(GCS_CHANNEL_WRITE_CHUNK_SIZE));
}

if (properties.containsKey(GCS_OAUTH2_TOKEN_EXPIRES_AT)) {
gcsOAuth2TokenExpiresAt =
new Date(Long.parseLong(properties.get(GCS_OAUTH2_TOKEN_EXPIRES_AT)));
}
```

In `AzureProperties.java`:
```java
if (properties.containsKey(ADLS_READ_BLOCK_SIZE)) {
this.adlsReadBlockSize = Integer.parseInt(properties.get(ADLS_READ_BLOCK_SIZE));
}
if (properties.containsKey(ADLS_WRITE_BLOCK_SIZE)) {
this.adlsWriteBlockSize = Long.parseLong(properties.get(ADLS_WRITE_BLOCK_SIZE));
}
```

When configuration properties are merged or populated from frameworks (e.g. Spring Cloud, Hadoop/Spark configuration maps, or REST client dictionaries) containing entries mapped to `null`, `properties.containsKey(...)` evaluates to `true`. Calling `Integer.parseInt(null)` or `Long.parseLong(null)` results in:

```
java.lang.NumberFormatException: Cannot parse null string
```

#### Expected Behavior

Optional numeric properties with `null` values in the configuration map should be treated safely as absent (`Optional.empty()`), consistent with how `PropertyUtil.propertyAsNullableInt` and `PropertyUtil.propertyAsNullableLong` operate across the rest of the codebase.

#### Reproduction

```java
Map properties = new HashMap<>();
properties.put(GCPProperties.GCS_CHANNEL_READ_CHUNK_SIZE, null);
new GCPProperties(properties); // Throws NumberFormatException: Cannot parse null string
```

#### Proposed Solution

Update `GCPProperties` and `AzureProperties` to parse optional numeric fields via `PropertyUtil.propertyAsNullableInt` and `PropertyUtil.propertyAsNullableLong`.

I have a working solution with regression tests ready in branch `Prabal864:fix/gcp-azure-properties-null-handling` and would be happy to open a PR for this once assigned.

### Willingness to contribute

- [x] I can contribute a fix for this bug independently

Start with GCPProperties.java and AzureProperties.java, then read PropertyUtil.propertyAsNullableInt and PropertyUtil.propertyAsNullableLong to compare the existing nullable parsing behavior. Reproduce the null-valued property cases and run the regression tests mentioned in the issue; done means both classes safely treat optional null numeric values as absent without changing valid-value parsing.

Written by the indexing model from the issue text.