onChangeThreshold crashes on mixed-type readings

# 🐞 Bug Report

### Affected Services [**REQUIRED**]
device-sdk-c (AutoEvent onChangeThreshold implementation)

### Is this a regression?

Unknown - appears to be a latent bug in the `values_exceed_threshold` function that would trigger when comparing mixed-type readings (numeric vs non-numeric) with `onChangeThreshold > 0`.

### Description and Minimal Reproduction [**REQUIRED**]
Sometimes the value of a parameter can be undefined caused by communication issues, this causes the code to crash, as only on the them needs to be true.

**Location:** `src/c/autoevent.c:61`
**Bug:** Incorrect logical operator causes null pointer dereference when comparing values of mixed types.

**Current code (WRONG):**
```c
if(curr_val_cast != NULL || prev_val_cast != NULL) // Line 61 - OR operator
{
double curr_val = iot_data_f64(curr_val_cast); // Line 63 - crashes if NULL
double prev_val = iot_data_f64(prev_val_cast); // Line 64 - crashes if NULL
```

**Fix:**
```c
if(curr_val_cast != NULL && prev_val_cast != NULL) // AND operator
```

**Reproduction:**
1. Configure AutoEvent with `onChangeThreshold > 0`
2. Device resource returns multiple readings with mixed types:
- Reading A: numeric (e.g., `42.5`)
- Reading B: non-numeric (e.g., `"ACTIVE"`)
3. Trigger AutoEvent
4. `values_exceed_threshold()` attempts `iot_data_f64(NULL)` → crash or undefined behavior

**Expected behavior:** Non-numeric values should fall through to `iot_data_equal()` comparison at line 72.

**Actual behavior:** Undefined behavior when calling `iot_data_f64()` with NULL pointer.

## 🔥 Exception or Error

Segmentation fault or undefined behavior when iot_data_f64(NULL) is called.

Exact error depends on iot_data_f64() implementation's null handling.

## 🌍 Your Environment
**Deployment Environment:** n/a

**EdgeX Version [**REQUIRED**]:** Latest main branch (commit 3f0e136)

**Anything else relevant:**
The else branch at line 71 correctly handles non-numeric comparisons using `iot_data_equal()`, but it's unreachable when values are of mixed types due to the OR operator bug.